Make difftools pass PEP8

Still want to add docstrings and refactor camel case names in this old
code.
Use astropy.stats.funcs.sigma_clip for diff comp

Use the robust astropy.stats.funcs.sigma_clip for cleaning bad pixels
rather than rolling our own sigma clipping.

Using sigma=2.5 by default, and iterating until convergence is reached.
_computeDiff reports unmasked coverage fraction
Save diff_path for difference FITS image to MosaicDB
ScalarMosaicFactor.solve_offsets has multiprocessing toggle

If mp_diffs=False, then differences images are computed in serial
(useful if computations are disk or memory bound).
Create diffplot module

- implement plot
- implement plot_diffs wrapper
Mask pixels in difference images for better offsets

ScalarMosaicFactory.solve_offsets() takes a block_mask_path argument
with BlockDB key for an optional mask image. This is propagated to
Couplings and SliceableImage to mask pixels before differencing.

This lets the user mask objects or image artifacts not covered by the
the weightmap's zeroed pixels (reflection halos are a good case).