Desc interface

Solves the following issue(s):

Closes #268 (closed).

Core changes:

• Added the new classes DESCunit and DESCequilibrium to interface to the DESC equilibirum code.
• Changed the abstract methods for LogicalMHDequilibrium from b2, j2 to covariant b1 and j1.
• Added caching to b1 and j1 evaluations of GVEC and DESC equilibria.
• Added Struphy unit conversion to GVEC and DESC equilibria.
• Added physical constants to units dictionary.
• Added current density unit to units dictionary.

Model-specific changes:

None

Documentation changes:

• Added the Tutorial 07 on MHD equilibria.
• Improved doc on Normalization