pass a 'const char*' to read_input_parser
libcvcpp: expand comments relating to propagator filename construction, adjust subdirectory name for multiple masses in non-MMS files
test_libcvcpp: add usage information and process command line arguments
fix typo introduced into serial read_lime_gauge_field_doubleprec
libcvcpp: add support for 'observables' for a given flavour pairing, these correspond to different numbers and types of gamma combinations, as defined in previous contraction codes