Refactor parsing of symbol lists

For symbol lists (such as those given to “var” or “stoch_simul”), return a
semantic value at the Bison level, rather than building the list through a
temporary object in ParsingDriver. This allows various simplifications, and
leads to a more functional code.

By the way, homogeneize the naming of the various flavours of symbol lists in
the Bison file (with TeX names, partitioning, or with a wildcard).