Update psf.py

We should set the fixed / not-fixed here in every call, since we don't know what state they are in before. Sure, the psf object is initialized with those values fixed, but they may have been changed in a first run of `psf_photometry`. Let's say I don't like the result and run `psf_photometry` again with `tune_coordinates=False` then I want them to be fixed, independent of the current value of `psf.fixed['x_0']`.
Add make_gaussian_sources
Add make_random_gaussians
Add Possion noise option
Doc improvements to new make_* functions
Add option to return table of random sources
Remove make_gaussian_image
Allow amplitude or flux inputs
Add test for make_gaussian_image
Return table from make_random_gaussians
Add x/ystddev_range
Add check_random_state from scikit-learn
Use check_random_state
Add discretize_model methods
Add make_noise_image
Use oversample discretization
Add apply_poisson_noise
Raise error in stddev is None
Add tests for new datasets functions
moving docstring of area() to the base Aperture class