Refactor FEEC initialization

Solves the following issue(s):

Closes #241 (closed)

Core changes:

FEEC variables are initialized as background + perturbation. Details can be found in a new section of the documentation (see below).

Moreover:

• Added bckgr_params and pert_params to create_field(); new types LogicalConst and MHD for background params keyword.
• Remove electric equilibrium
• Added unit tests for new init

Model-specific changes:

None

Documentation changes:

New sections sections/subsections/initial_conditions and sections/subsections/fluid_backgrounds.