Tracking issue for improvements to astropy.modeling

[astrofrog]

Just some notes so I don't forget!

• Need to fix Gaussian1D so it doesn't actually use bounds: https://github.com/astrofrog/astropy/tree/remove-bounds-functional - fails currently because can't pickle the wrapped getters and setters, and also seems to be a numerical issue in the fitting, which is strange.
• Branch with deprecation of LMLSQFitter for models with bounds: https://github.com/astrofrog/astropy/tree/deprecate-bounds-lmlsqfitter
• Need to deprecate use_min_max_bounds in TRFLSQFitter and DogBoxLSQFitter