Pass constant string arguments by reference

src/include/Configuration.h

src/include/Parsers.h

src/include/TransitionMatrix.h

src/include/errors.h

src/include/file_io.h

src/include/logging.h

src/include/tfrfme.h

src/libnptm/Configuration.cpp

src/libnptm/Parsers.cpp

src/libnptm/TransitionMatrix.cpp

src/libnptm/file_io.cpp

src/libnptm/logging.cpp

src/libnptm/tfrfme.cpp