generate_levels_constraint takes any iterable coordinate, should we generalise this constraint?

[daflack]

The generate_levels_constraint works with any iterable coordinate, i.e. it takes model/pressure level and stratifies based on those. However, it also has the ability to stratify by ensemble member as realization is an iterable coordinate.

To close #132 the documentation was updated for generate_levels_constraint to show this. I took this option as I thought it was more sensible to document it than create duplicate code. However, this did raise a question: for ease of usability should the generate_levels_operator be renamed to something generic and the levels variable be renamed as well, or potentially just one of the two?