variable modification API modified

src/pappsomspp/peptide/peptide.cpp

src/pappsomspp/peptide/peptide.h

src/pappsomspp/protein/peptidefixedmodificationbuilder.h

...pappsomspp/protein/peptidevariablemodificationbuilder.cpp

src/pappsomspp/protein/peptidevariablemodificationbuilder.h

test/test_protein.cpp