Draft: Streamlined MAGMA2 implementation

This is a reimplementation of the MAGMA2 branch with:

• Simpler symmetric matrix handling
• Only 1st order reconstruction of v and u.
• Only basic viscosity and diffusion terms as in the Rosswog 2020 implementation

Questions:

• Do we need more advanced terms? SPHENIX-like? Or the Rosswog entropy-based viscosity switch?
• What implementation do we want? Traditional or Gasoline?

Todo:

• Document the matrix stuff
• 1D and 2D cases
• Scale-factors
• Strange corner cases (e.g. no ngb)
• Tests in galaxy settings
• Properly implement the symmetric functions.
• Fix documentation. It still mentions Minimal and SPHENIX in places.