Bug fix: allow unknown units in AVALU and DOSEU

[js3110]

Issue

Closes #836

Description

If units are not known by the units package, the app was crashing due to the simplify_units function for simplifying the volume units. I have updated this function to return a warning rather than stop if the units are not recognised, so that more non-traditional units (eg cells) can still be used.

Definition of Done

• App does not crash if non traditional units are used
• Gives warning that simplification was not possible

How to test

Using dummy_RO_cells in testthat/data folder, run through and check that the app doesn't crash with the units, or try your own dataset with different units.

Contributor checklist

• Code passes lintr checks
• Code passes all unit tests
• New logic covered by unit tests
• New logic is documented
• App or package changes are reflected in NEWS
• Package version is incremented

Notes to the Reviewer

I have added the dummy dataset to make it easier to test, we can remove it before I merge or keep it for testing purposes, let me know what you think is best :)