Improvements to Ref

[ccoffrin]

This is a refactor on how the ref data structure is built, with the goals of improved model build times and allowing problem formulations to extend the precomputed values before the post method is called.

Notable changes,

• The GenericPowerModel constructor now only builds a very basic version of ref, which does the fully automatic and generic transformation steps of data into ref. See build_generic_ref
• core_ref!(pm) can be used to populate ref with the basic info that nearly all models will need. It is called after the GenericPowerModel construct and before the model post method in run_generic_model.
• problem specific run methods can optionally add functions to the list defined by ref_extensions, which will be executed after core_ref!(pm) and before post.

The problem formulations OTS, TNEP, and OPB provide example for how this feature can be used. For example see, ne_branch_ref!.

On a slightly tangential point, also updated how the dcline variables are setup to remove the need for arcs_dc_param and further simplify core_ref!.

Comments, questions and suggestions are welcome. This will be merged shortly after the MOI transition is made.

CC @frederikgeth, @hakanergun @rb004f, @pseudocubic, @sanderclaeys, @kaarthiksundar

[ccoffrin]

@pseudocubic any particular reason to close this?