Define a coreneuron NMODL API

[ohm314]

@alkino had also mentioned this. Currently, NEURON/CoreNEURON mechanisms access functions and data from CorNEURON without any clear seperation between the codes and no clearly defined interface. We should define what is the interface of classes and functions of CoreNEURON that is used by NMODL and create an API. This allows us to more easily change internals while keeping the interface to NMODL stable, or at least make changes that might break the interface obvious.