MatchVector, AnnotationMatch, TokenMatch are now MatchData
MatchData is value_type of Match which now has get_value/set_value
Match has also copy construction working properly now
Default value for Match holds MatchVector so adding $m:_M var in grammar works as intended.

emphasise that parseStringOperator(const std::string&) parses a string, not a file, by calling the argument appropriately

The prefix like "$m:" signifies type in WCCL, but in Variables code it comes from template argument; in this particular example, the <Match> type.

add SentenceContext::get_sentence_ptr to allow acces to the senetnce shared_ptr that's already there anyway