Restructure the directories and cosmetic updates on README

[jdlph]

Besides the directory changes and README updates, there are some functionality updates including,

1. Adopt teestream.h for logging (i.e., simultaneously output log to terminal using std::cout and to local log.txt using std::ofstream);
2. Replace non-standard #pragma once with include guard;
3. Remove MathLibrary.cpp (which was removed) from CMakeLists.txt;
4. Update CMakeLists.txt to specify C++ standard(i.e., C++11);
5. Change argument list order in network_assignment() to be consistent with its counterpart in Path4GMNS;
6. Provide executable of DTALite on macOS and move all executables to bin.

[xzhou99]

confirm with the changes.