Rework matrix assembly + Hall/viscosity/v01/B01 extensions

[n-claes]

PR description

This PR includes a complete overhaul of the way the matrices are assembled. Every physical effect has been split off in its own dedicated submodule which handles matrix assembly only for those particular physics. Addition of these effects to the "main" matrices now depends on which physics are included instead of adding all zeroes. The boundaries are treated in a similar way, with a dedicated submodule for each physical part of the essential/natural boundary conditions.

This also adds two new physical effects to the code: viscosity and Hall MHD. The "allowed" equilibrium is also extended, since we can now have a non-zero v01 component allowing for compressible background flows on top of a non-zero (constant) B01 field allowing for magnetic fields aligned with the direction of inhomogeneity.

Regression tests + the way eigenfunctions are handled in the testing framework have been improved as well.

New features

• Allow for a non-zero v01 component
• Allow for a non-zero constant B01 component
• New physics: viscosity
• New physics: Hall MHD
• Allows to specify a custom grid
• When running in debug mode, Legolas will automatically save the cooling/atmosphere curves for inspection with Pylbo

Note: viscosity, v01 and B01 have been roughly checked and should work for the most part. We still have to add tests for these, but we can do that in a different PR (otherwise this one would get really big). Hall does NOT yet work properly, so for the moment warnings are printed whenever Hall/viscosity is enabled.