Rename cell type variable

[jhdark]

Proposed changes

With this change, the variable OF_cell_type_value has been simplified to cell_type, however as this isnt the clearest naming, a setter has been added to ensure the value given is an integer. Furthermore additional documentation has been added pointing users to where more information can be found on the number scheme for different types of cells

Types of changes

What types of changes does your code introduce to foam2dolfinx?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.75%. Comparing base (cf41b70) to head (7d35711).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
+ Coverage   97.53%   97.75%   +0.22%     
==========================================
  Files           2        2              
  Lines          81       89       +8     
==========================================
+ Hits           79       87       +8     
  Misses          2        2              

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