getHalfHaldaneSub might have issues for large number of relationships

[dczielinski]

getHalfHaldaneSub tries to create an equivalence relationship to substitute out a rate constant algebraically based on First Law constraints. When many First Law constraints are present for an enzyme, these are provided by a list. For each element in the list, we get a list of rate constants. For each rate constant, we see if it has already been subbed out and if not we create an equivalence relationship for the first one. The potential issue is that having too many pre-existing relationships such that all rate constants have been previously substituted and no new rate constant relationships can be added. This is likely an extreme fringe case, but it may be possible to create safer behavior with quite a bit more complicated checking and storing of results from previous rounds (i.e. store all equivalence relationships, then solve at the very end for independent rate constant substitutions to use).