Reduce dependencies in core modules

It would be good to reduce, as much as possible, the dependencies required by the "core" modules (in `./unseen`). When we run the tests in github Actions, we have to create the environment each time, so minimising the complexity of this environment is beneficial. E.g. `general_utils` contains `import matplotlib as mpl`. Is this really necessary, or shall we decide that plotting is beyond scope?

Start by auditing imports across the core modules in ./unseen, beginning with unseen/general_utils.py and its matplotlib import. Compare those dependencies with the GitHub Actions test environment and identify which are unnecessary for core use. Done means the dependency scope is agreed, the environment is simpler, and the full test suite still passes.

Written by the indexing model from the issue text.