Unify equilibrium representations

[mattngc]

Currently there are two equilibrium representations used in different parts of the code, Fiesta() and Cherab's EFITEquilibrium. Both have essentially the same underlying data and interface requirements.

Suggest merging both of these into a new customised VitaEquilibrium object with class methods for creation. E.g. .from_fiesta() and .from_cherab(). This will simplify the code substantially.