Mesh normal as property of F.Mesh

[jhdark]

Proposed changes

New property of the mesh which will allow for evaluating vectors normal to boundaries of the mesh for example.

Types of changes

What types of changes does your code introduce to FESTIM?

• 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

• Black formatted
• 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 lines are covered by tests ✅

Comparison is base (4836eba) 95.83% compared to head (4fb8584) 95.88%.

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #605      +/-   ##
===========================================
+ Coverage    95.83%   95.88%   +0.04%     
===========================================
  Files           10       10              
  Lines          264      267       +3     
===========================================
+ Hits           253      256       +3     
  Misses          11       11              

Files	Coverage Δ
festim/hydrogen_transport_problem.py	97.50% <100.00%> (-0.03%)	⬇️
festim/mesh/mesh.py	100.00% <100.00%> (ø)

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