Fix non-deterministic ShapeError in 3D FEM gradient method

[Rishab87]

Description

This PR resolves a non-deterministic ShapeError that occurred in the ScikitFiniteElement3D spatial method when simulating 3D thermal models.

The problem was when evaluating the gradient of a constant parameter, like thermal conductivity (lambda_eff), which PyBaMM simplifies to a pybamm.Scalar. The gradient method was not designed to handle a scalar input and would attempt an invalid matrix-scalar multiplication.

Fixes #5142

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 99.08%. Comparing base (d421e3a) to head (e844170).
⚠️ Report is 163 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5143   +/-   ##
========================================
  Coverage    99.08%   99.08%           
========================================
  Files          310      310           
  Lines        24459    24459           
========================================
  Hits         24236    24236           
  Misses         223      223           

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[aabills]

Great, thanks!!