GriddablePSFModel does not accept 2D oversampling

[Onoddil]

Unlike FittableImageModel, GriddablePSFModel doesn't accept 2D oversampling (in the form of an (x, y) tuple). Presumably this should be extended to match the parameters accepted by the base class?

It also appears in the docstring to want an integer oversampling factor, but FittableImageModel accepts float or tuple of two floats, and GriddablePSFModel only checks for isscalar for error raising purposes.