grid_number_density should accept custom models

[dncnwtts]

I would like to make small changes to the sky model and output 3D grids as a function of these parameters. Unfortunately, the call for grid_number_density requires the name of a built-in model.

I would like it to be possible for a custom ipd_model to be an argument, in case the user has this. So basically, either name or ipd_model could be an argument.

One idea we have had is adding new models to the registry, but that would be cumbersome in the case of trying to make a list of ~1000 different parameters.

[MetinSa]

Good suggestion! I suggesting it accepting either a str, and if so it is a built-in model, or a custom model object