Feature/example of drop=True

[RobbieKiwi]

Closes #464

Changes proposed in this Pull Request

Add an example to the docs of LinearExpression.where(..., drop=True)
Add new property to base expression BaseExpression.variable_names

Not addressed / further ideas

In the issue mentioned, I think the reporter wanted to see the unused terms in LinearExpression.__repr__. After having a bit of a look at it, I think this is impossible because the term dim has no coordinate names. There seems to no way to know what variable originally belonged in a position of the term dim if the original variable has been completely masked (replaced with -1).

Maybe when drop is True we could also run .simplify to get rid of any empty positions in the term dim?
Currently drop=True will drop unused index positions along all dimensions apart from the term dim, it might make more sense to drop on all dims?

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[FabianHofmann]

thanks for the pr @RobbieKiwi, one comment

[FabianHofmann]

I don't think nvar is a good name here. in model.nvar this relates to the number of reference variables (each single element in a variable array). perhaps let's remove this function for now?