Update GVEC interface and 3D polar spline test
- Add contravariant implementations to
Equilibrium_mhd_gvec
. - Implement dummy bulk density, imitate
Equilibrium_mhd_physical
. - To make GVEC interface compatible with
lin_mhd.py
. - Implement equilibrium current for GVEC interface.
- Update 3D polar spline test.