[enhancement] Improve user-interface for generating models

[TomHilder]

The interface should be changed so that it does not require writing and reading files at all. The output data of wakeflow is small enough to be sensibly kept in memory, for instance for use in a self-contained script, or an Jupyter notebook for analysis.

This is sort of already implemented in wakefit_interface.py, but it’s not done all that well and also is not accessible to the user easily right now.

This would be a relatively straight-forward change, but it should be left until after the new features from #21 have been merged.