Add type parameters to choose formulations flexibly per device

[coroa]

Fixes #11.

• Added a new hierarchy of formulations for devices and extended the energymodel parameters with a powermodels formulation type.
• Rewrote generator.jl, link.jl

Features

• Separation of investment and dispatch formulations
• Possibility to split times and locations (through maintaining axes on the EnergyModel)

TODO

• [x] Allow data to customize the formulations, ie p_nom_extendable -> LinearExpansionFormulation
• [x] Store axes, jump model and variables on the EnergyModel, instead
• [x] Add a parent attribute referring optionally to a second EnergyModel for the StructJuMP capability
• [x] Add model-centric Formulation change
• [x] Hook up demote_formulation choices
• [x] Rewrite all devices:
  • [x] Generator
  • [x] Link
  • [x] Line
  • [x] StorageUnit
  • [x] Transformer
  • [x] Store
  • [x] Bus
  • [x] SubNetwork