observable table vs observables table

[FFroehlich]

Documentation is inconsistent in using "observables table" and "observable table" https://petab.readthedocs.io/en/latest/documentation_data_format.html#observables-table

Code seems to be pretty consistent with "observable_df", so maybe we should stick with that?

[dweindl]

Agreed, would be good to use consistent terminology. No strong preference for either "observables table" or "observable table".