Fix Cavity being broken by vectorisation

[jank324]

Description

Re-adds the use of base_rmatrix for Cavitys with voltage=0.0 because the usually cavity matrix breaks in that case. This was originally removed with the vectorisation because it was difficult to integrate and though to be unnecessary.

Motivation and Context

Closes #214.

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Types of changes

• Bug fix (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 change)
• Documentation (update in the documentation)

Checklist

• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code and checked that formatting passes (required).
• I have have fixed all issues found by flake8 (required).
• I have ensured that all pytest tests pass (required).
• I have run pytest on a machine with a CUDA GPU and made sure all tests pass (required).
• I have checked that the documentation builds (required).

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[cr-xu]

The failing test is actually related to the broadcast defaulting to float32 #217

[jank324]

The failing test is actually related to the broadcast defaulting to float32 #217

I've merged #218 and now they are passing.