Follow-up polishing from #540 (Object oriented refactoring of ionic models and plotting calcium)

[michelebucelli]

Follow up to threads from #540 that were not resolved before merging.

• Moving ionic model parameter classes to ionic_model.h/ionic_model.cpp, to improve decoupling between ionic models and the rest: original thread.
• Remove code duplication between ParameterLists and ionic model parameter classes: original thread.
• Identifying ionic models only through either std::strings or the enum class ElectrophysiologyModelType, but not a mix of both: original thread.