omit conversion to mass action for stoichiometries > 100 to avoid StackOverflow in simplify_fractions

[paulflang]

Checklist

• [ x] Appropriate tests were added
• [x ] Any code changes were done in a way that does not break public API
• [x ] All documentation related to code changes were updated
• [x ] The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• [x ] Any new documentation only uses public API

Additional context

This is to fix #160 . The model in question contains stoichiometries in excess to 1e4, which causes StackOverflow in SymbolicUtils.simplify fractions. The fix here is to no longer attempt to recover mass action kinetics from generic SBML rate laws whenever reactant stoichiometry is >100. This circumvents the need for calling simplify_fractions.