Suppress NaN warnings

[wmwv]

We get NaN warnings from two things that we expect to happen:

1. NaN regions of images:

WARNING: Input data contains unmasked non-finite values (NaN or inf), which were automatically ignored. [photutils.psf.photometry]

2. Negative flux values converted to magnitudes:

mk_lc.py:208: RuntimeWarning: invalid value encountered in log10
  star_fit_mags = -2.5 * np.log10(xm["flux_fit"])

Add code to suppress this warnings. Either for everything or just as specific context managers.