silencing compiler warnings

[andrewdavidsmith]

• EpireadStats: removing constexpr as it's not always supported yet for the way it was used
• bsrate: adding to_string around an integer that was concatenated to a string
• MSite.cpp: putting a guard around use of from_chars because on osx it is not yet available from c++17
• amrfinder.cpp: ensuring move is std::move because it must be explicit fir some compilers. Also casting properly for size_t seeming to be unsigned long long on apple
• Removing unused variables to silence warnings. In some cases these seem to have been important
• amrfinder: Fixing a warning issues by using decltype in a static cast
• ThreeStateHMM: putting guards around backward score as it's only used to compare with forward score in debug situation within assert
• Fixes #169.
• pmd: many places the vector indicating array_status had been passed around despite only being used in a few of those places. These were deleted