Improve code coverage

[bobmyhill]

Coverage as of 31/10/2025 is 93.6% (https://github.com/geodynamics/burnman/actions/workflows/coverage.yml)

Much of the "untested" code is tested through the examples and benchmarks, but it would be good to unit test the whole codebase.

Coverage reports are generated as a Github Action. To find the most recent report, click the tick button opposite the most recent commit, find the CodeCoverage action, and then click "Summary". A link to the artifact should be at the bottom of the page.

Note that coverage.py doesn't work on functions with numba decorators (coveragepy/coveragepy#849). The Github Action uninstalls numba for coverage testing.