Override boundary value and provide constant extrapolation

[aabills]

Description

Allows for constant extrapolation of boundary values (i.e. using the value of the last node) and allows the user to override the spatial method's extrapolation method. This is useful for geometric input parameters, if one wants to calculate the harmonic mean of the diffusivity at the interface. A follow up PR (separating because they are pretty unrelated) will finish this capability by creating a node in the expression tree to retrieve the mesh size at the boundary.

Fixes # (issue)

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.10%. Comparing base (f04a1f8) to head (54916e2).
⚠️ Report is 177 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5107   +/-   ##
========================================
  Coverage    99.09%   99.10%           
========================================
  Files          306      306           
  Lines        23659    23671   +12     
========================================
+ Hits         23446    23458   +12     
  Misses         213      213           

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[MarcBerliner]

Thanks!