Some reorganization of the PSF subpackage

Include setup_requires in package setup

Use define minimum python and numpy versions in install docs

Add a PSF model for spatially-dependent PSFs

Prevent pytest from picking up test function as a test

Update astropy_helpers to v3.1rc1

Pin pytest<3.10 for astropy LTS travis test

Do not use deprecated _naxis1/2 for astropy<3.1

Update find_peaks to return empty table with column names if no peaks are found

Note that an empty table is returned if no sources are found

Make table output of DAOStarFinder and IRAFStarFinder consistent

Allow for masking regions in star finders

Added doctest skip to matplotlib import line in aperture.rst

Add filtering/selection by peakmax and brightest

Creation of initial template PSF block diagram documentation files

Raise errors for invalid inputs to star or peak finders

Add docs for custom local background estimation

Remove warnings about ePSF smoothing kernels

Check that the input ePSF fitter is an EPSFFitter instance

Making deprecations API consistent

Update .gitignore

Add warnings for using a bad smoothing kernel/oversampling combo

Fix typos in detection docs

update terminology on discussion from PSF workshop

Rename "Star" classes with EPSF prefix