Fixed DESCequilibrium

Solves the following issue(s):

Close #273 (closed)

Core changes:

• In DESCequilibrium.desc_eval, evaluate the contra-variant components B^rho, B^theta, B^zeta and J^rho, J^theta, J^zeta, respectively, instead of the now used covariant components. The covariant components seem to have a bug.
• Demand the contra-variant bv and jv instead of the covariant b1 and j1 in LogicalMHDequilibrium.
• Adapt GVECequilibrium accordingly.

Model-specific changes:

None

Documentation changes:

None