Resolve "Add special membranes for species that only pass through to it but not diffuse"

What does this MR do?

• Add a Finite Volume (FV) local operator.
• Re-structure the pdelab tree to allow both, Finite Volume as well as Conforming Finite Elements.
  • Make stand-alone diffusion-reaction model for CG work again
  • Make a stand-alone diffusion-reaction model for FV
    • Add similar test to what is already in multidomain (or improve them e.g. test agains analytic functions with an L2 error integration)
    • Write cell data by dafault
  • Make a multidomain diffusion reaction model with only FV
  • Manage domains with no variables by mapping domains to gfs index (yet another level of indirection)
  • Make a stand-alone diffusion-reaction model with composite FV and CG
  • Make a multidomain diffusion-reaction model with composite FV and CG
  • Use the virtual interface of local finite elements to choose between FV and CG discretization
• Allow selective refinement per compartment i.e. anisotropic cube refinement.

As part of the work for this MR, issues #15 (closed) and #19 (closed) was solved here

Is there something that needs to be double checked?

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Can this MR be accepted?

• Implemented
• Added/Updated tests:
• Added/Updated documentation
• Pipelines passing
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• Delete branch option set
• Added entry to CHANGELOG.md

Related issues

Closes #20 (closed), #15 (closed). #19 (closed)