Specutils model fitting fails to handle slope parameter units

In fitting.fitmodels, [_add_units_to_model()](https://github.com/astropy/specutils/blob/feb7050c3007ad49260e99df55923dc0f69d513e/specutils/fitting/fitmodels.py#L690) throws an error when trying to handle a slope parameter in e.g. a 1D linear model, since the slope parameter has units of [y units / x units] and thus fails both of the current criteria (input parameter units match either x units or y units).

Start in specutils/fitting/fitmodels.py at _add_units_to_model() around line 690 and trace how units are checked for fitted parameters. Reproduce the failure with a 1D linear model whose slope has y/x units. Done means slope parameters with combined units are accepted without the reported error.

Written by the indexing model from the issue text.