Function to remove a single user defined unit

[jl-wynen]

Given that you are planning to add more features to user defined units as mentioned in #268 (comment), it would be nice for our use case to add the ability to remove a single user defined unit.

Currently, we implement this functionality by clearing all user defined units and re-adding all except the removed one. Performance is not curial for us, so it works fine. But I would expect other users to face a similar problem.

[phlptp]

Noted: I will look into it shortly. Thanks for the suggestion

[jl-wynen]

Thanks!