Start with psydac tests and MHD operators
- Added psydac tests: test_psydac_basics.py, test_psydac_parallel.py, test_psydac_lin_ops.py.
- Adapted DOMAIN class to incorporate Psydac mappings.
- Adapt parameter names for cuboid mapping in all input files.
- New code lin_mhd_psydac: initialization of mhd variables as stencil vectors done.
- ADDED: test for spline basis indiexing, span.
- Mention manual install of gvec_to_python and psydac in README and doc.
- ADDED: in 1d projectors global, knot span index and splines evaluated at quadrature points in format (i, iq, basis function).