removed solution_formulae and renamed formulas to site_formulae

[bobmyhill]

Removes SolutionModel().solution_formulae object and renamed SolutionModel().formulas to SolutionModel().site_formulae.

The SolutionModel().solution_formulae object was originally intended to strip the sites from each site formula in the SolutionModel and return the chemical formulae, e.g.:
[Mg]3[Mg0.5Si0.5]2Si3O12 -> Mg4Si4O12
This intended use is essentially a duplication of the Solution().endmember_formulae object, which is itself derived from SolutionModel().endmembers. So the intended functionality is provided by another object.

Since writing the function, solution models have been generalised to the point that SolutionModel().solution_formulae might look quite different from Solution().endmember_formulae, but would not be as readable or contain any information that is not already in SolutionModel().formulas (now SolutionModel().site_formulae). For example:
Mg3Fef2Si3O12 is no more useful than [Mg]3[Fef]2Si3O12.