[WIP][md] Implement box staggered coupling

Todos:

• Add test for segment-segment intersection algorithm
• Make segment-segment intersection algorithm more efficient
• Put segment-segment in different MR
• Extract Box-Forchheimer to separate MR
• New BJ test (existing test uses indefinite perm. matrix)
• Change convergence script to specify convergence rate
• It currently only works for DiffusionCoefficientAveragingType::ffOnly
• Maxwell Stefan Diffusion law not yet implemented when using Box
• Renaming: Use ff/pm instead of stokes/darcy
• New IC assume specific parameter group "Darcy", generalise this
• Add missing tests

Suggestions:

• Currently, we retrieve the entire context via couplingContextVector(), but the old interface is still there. I would shrink this to only one interface and probably rename it. Currently, there is one context object per coupling segment, so maybe we can find a better name.
• At the moment the coupling segment geometry is stored in both the darcy and the stokes coupling info objects in the mapper. I don't think this is the most memory consuming part of the code, but there is room for memory efficiency improvements.

Fixes #788 (closed).