Support PSFEx variable psf

[navii98]

Hi,

It would be nice if support for PSFEx psf file can be added in photutils (at least in limited capacity).

The structure of the variable psf file is a fits files containing two extensions and the structure is as follows -
The 2nd extension (hdul[1]) contains the relevant header and PSF data.

The header contains the following import parameters -

1. PSFvariation_dependent variable (say x, and y)
2. variation degree
3. Oversampling factor (float)

The data is an array shaped (1, 1, n, psf_x, psf_y) where n is the coefficient for the polynomials.
e.g. for a 3rd order variation, n=10.
The coefficients, according to the variation degree are in the following order -
vardeg=0 -> [1]
vardeg=1 -> [1, x, y]
vardeg=2 -> [1, x, x2, y, y*x, y2]
vardeg=3 -> [1, x, x2, x3, y, yx, yx2, y2, y*x2, y3]
and so on.

The variation can depend on any image parameter (e.g. flux) or any other fits parameter (e.g. temperature, focus) but that might be difficult to implement. Therefore, I am suggesting a limited support that just covers x and y coordinates of the image.

I looked around the code, and It could just be a subclass of the FitttableImageModel. We would need to implement a new `evaluate' method.
For every x_0 and y_0 we can calculate a coefficient array -> multiply it with psf_array -> sum all arrays together to get the final PSF.
However, I am unsure how it will affect the other internal function of the class.