Allow bareword keys in nested action-arg hashes

BNGL inherits Perl's `=>` fat-comma semantics, so action arguments like
`generate_network({overwrite=>1,max_stoich=>{R=>6}})` are syntactically
valid even though the inner hash key `R` is unquoted. The pyparsing
grammar in `ActionList.define_parser()` required `quote_word` for
nested-curly keys, so any model that relied on bareword keys (notably
`max_stoich=>{<molecule>=><N>}`) failed at parse time before BNG2.pl
was ever invoked.

Accept either bareword or quoted keys inside nested hashes:

    curly_arg_token = (base_name ^ quote_word) + "=>" + arg_type_int

The existing dict-handling path in `BNGParser._parse_action_line` rebuilds
the literal substring verbatim, so round-trip via `Action.gen_string()`
is unchanged.

Author: wshlavacek

bionetgen/core/utils/utils.py

@@ -487,7 +487,10 @@ def define_parser(self):
         arg_type_list = "[" + pp.delimitedList((quote_word ^ arg_type_float)) + "]"
         arg_type_string = quote_word
         #
-        curly_arg_token = quote_word + "=>" + arg_type_int
+        # BNGL/Perl `=>` auto-quotes its left operand, so dict keys
+        # may be either bareword (max_stoich=>{R=>6}) or quoted
+        # (max_stoich=>{"R"=>6}). Accept both.
+        curly_arg_token = (base_name ^ quote_word) + "=>" + arg_type_int
         # TODO: handle 0 case
         arg_type_curly = "{" + pp.delimitedList(curly_arg_token) + "}"
         arg_types = (