[TODO] Add units into SherpaFitter.

[nocturnalastro]

As units are coming to astropy.modeling we should probably discuss how we should implement them into saba at some point. The example I tried doesn't work, as saba removes all quantity information.

I think this should be a relatively simple case of modifying the fitter_unit_support decorator which converts all inputs into the same unit system, strips the units and then converts the units back at the end.
Our use case is fairly unique. I think as only our fitter deals with multiple models/datasets and errorbars but I think the above decorator is a good starting point.

I haven't looked into it much yet so I'll update it when I do.

[nocturnalastro]

I've started working on this I'll finish off the tests next weekend and I'll make a PR.