Resolve "Custom membrane flux"

What does this MR do?

This MR adds parsers for reactions that happen on the intersection between two domains. Most of the new code is devoted to set up the parsers and its variables. The jacobian pattern is also updated so that only the required off-diagonal entries are filled on the matrix.

Is there something that needs to be double checked?

Does this work in the GUI? @liam.keegan

Can this MR be accepted?

• I've read contribution guide
• Implemented
  • Parse expressions for membrane fluxes
  • Compute fluxes based on the membrane expressions
  • Fill pattern according to the membrane flux
• Added/Updated tests
  • Update multidomain test to the new sections in the ini file
• Added/Updated documentation
  • Added new sections on the parameter tree description
• Pipelines passing
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  • 
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• Delete branch option set
• Fast-forwaded git merge is possible
• Added entry to CHANGELOG.md

Related issues

Closes #36 (closed), #54 (closed)