Fix compiler warnings

[ivorysoap]

/usr/include/c++/8/bits/stl_uninitialized.h:76:9: note: parameter passing for argument of type ‘std::move_iterator<nlohmann::basic_json<>*>’ changed in GCC 7.1
serialMonitor.cpp: In member function ‘std::__cxx11::string SerialMonitor::getData()’:
serialMonitor.cpp:36:44: warning: narrowing conversion of ‘str1.std::__cxx11::basic_string<char>::find('<', 0)’ from ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} to ‘int’ inside { } [-Wnarrowing]
                 int str1pos[3] = {str1.find('<'), str1.find('>'), str1.length()};
                                   ~~~~~~~~~^~~~~
serialMonitor.cpp:36:60: warning: narrowing conversion of ‘str1.std::__cxx11::basic_string<char>::find('>', 0)’ from ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} to ‘int’ inside { } [-Wnarrowing]
                 int str1pos[3] = {str1.find('<'), str1.find('>'), str1.length()};
                                                   ~~~~~~~~~^~~~~
serialMonitor.cpp:36:78: warning: narrowing conversion of ‘str1.std::__cxx11::basic_string<char>::length()’ from ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} to ‘int’ inside { } [-Wnarrowing]
                 int str1pos[3] = {str1.find('<'), str1.find('>'), str1.length()};
                                                                   ~~~~~~~~~~~^~
serialMonitor.cpp:37:44: warning: narrowing conversion of ‘str2.std::__cxx11::basic_string<char>::find('<', 0)’ from ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} to ‘int’ inside { } [-Wnarrowing]
                 int str2pos[3] = {str2.find('<'), str2.find('>'), str2.length()};
                                   ~~~~~~~~~^~~~~
serialMonitor.cpp:37:60: warning: narrowing conversion of ‘str2.std::__cxx11::basic_string<char>::find('>', 0)’ from ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} to ‘int’ inside { } [-Wnarrowing]
                 int str2pos[3] = {str2.find('<'), str2.find('>'), str2.length()};
                                                   ~~~~~~~~~^~~~~
serialMonitor.cpp:37:78: warning: narrowing conversion of ‘str2.std::__cxx11::basic_string<char>::length()’ from ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} to ‘int’ inside { } [-Wnarrowing]
                 int str2pos[3] = {str2.find('<'), str2.find('>'), str2.length()};
                                                                   ~~~~~~~~~~~^~
serialMonitor.cpp:39:32: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} [-Wsign-compare]
                 if( str1pos[0] != string::npos && str1pos[1] != string::npos && str1pos[0] < str1pos[1] ) {
                     ~~~~~~~~~~~^~~~~~~~~
serialMonitor.cpp:39:62: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} [-Wsign-compare]
                 if( str1pos[0] != string::npos && str1pos[1] != string::npos && str1pos[0] < str1pos[1] ) {
                                                   ~~~~~~~~~~~^~~~~~~~~
serialMonitor.cpp:44:37: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} [-Wsign-compare]
                 else if( str1pos[0] != string::npos && str2pos[0] != string::npos && str2pos[1] != string::npos ) {
                          ~~~~~~~~~~~^~~~~~~~~
serialMonitor.cpp:44:67: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} [-Wsign-compare]
                 else if( str1pos[0] != string::npos && str2pos[0] != string::npos && str2pos[1] != string::npos ) {
                                                        ~~~~~~~~~~~^~~~~~~~~
serialMonitor.cpp:44:97: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} [-Wsign-compare]
                 else if( str1pos[0] != string::npos && str2pos[0] != string::npos && str2pos[1] != string::npos ) {
                                                                                      ~~~~~~~~~~~^~~~~~~~~
serialMonitor.cpp:49:37: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} [-Wsign-compare]
                 else if( str1pos[0] != string::npos && str1pos[1] != string::npos && str2pos[1] != string::npos ) {
                          ~~~~~~~~~~~^~~~~~~~~
serialMonitor.cpp:49:67: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} [-Wsign-compare]
                 else if( str1pos[0] != string::npos && str1pos[1] != string::npos && str2pos[1] != string::npos ) {
                                                        ~~~~~~~~~~~^~~~~~~~~
serialMonitor.cpp:49:97: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘unsigned int’} [-Wsign-compare]
                 else if( str1pos[0] != string::npos && str1pos[1] != string::npos && str2pos[1] != string::npos ) {