Added load_mesh_from_file function to support external 3D meshes.

[Rishab87]

Description

Added helper functions to import external 3D meshes generated from other tools or libraries which can be solved in PyBaMM. This can help users get high quality meshes from other libraries which we can't directly support in PyBaMM due to licensing issues.

Note: I've only tested it for .msh, .xdmf file formats which are the most commonly used ones. But it may support several others.

Related to #5104

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #)

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[rtimms]

looks great, thanks @Rishab87 !

[codecov]

Codecov Report

❌ Patch coverage is 93.23308% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.86%. Comparing base (4a875e9) to head (7fc6801).
⚠️ Report is 159 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/pybamm/meshes/scikit_fem_submeshes_3d.py	93.23%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5162      +/-   ##
===========================================
- Coverage    98.89%   98.86%   -0.03%     
===========================================
  Files          320      320              
  Lines        26435    26568     +133     
===========================================
+ Hits         26142    26266     +124     
- Misses         293      302       +9     

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[rtimms]

@Rishab87 can you improve the coverage then this is g2g

[Rishab87]

@rtimms I've ensured coverage I think it should be fine now, can you re-review it?