processExposure: ensure cache is pure for each iteration
background: tweak score weights, and document each score

Placing higher emphasis on filling the area.  This calculation could be
done better with the new CartesianPolygon class.
background: parallelise exposure data loading (for overlaps, etc).
stack: make warp optional

Saves descending into lots of patches only to discover nothing needs to
be done.
stack: add background subtraction of final coadd
background: fix x,y bug in RadiusTaperWeightImage

Tapers for HSC were really bad because x,y were transposed.
background: remove config variables in sub-config.

undersampleStyle and interpolation are background.undersampleStyle and
background.algorithm, respectively.
background: no tapering needed (or wanted) when there's a single input

If we taper, then we lose pixels where the taper = 0.
background: move background merging into MatchBackgroundsTask

MatchBackgroundsTask generated the background models, so it (and only it)
needs to know how to merge them.
background: expand debugging outputs
background: remove patches with no overlap

Having them in causes trouble, and they'll not be missed.
background: adjust some subtleties in reference construction

When to zero out bad pixels and when not to is a fine art, it seems...
stack: fix background subtraction issues

Added note about background subtraction, fixed output.
Extensive updates for background matching coadds

* New parallel (MPI-based) task to assemble coadds (with scatter-gather to
  merge background models for background matching).
* Background models implemented in python, including:
  + A re-implementation of lsst.afw.math.BackgroundMI (with over-sampling,
    support for polygons)
  + A chebyshev polynomial model
  + An empirical multi-polygon model
  + Built-in support for merging models from different areas
  These should be pushed down to C++ eventually, but they have been useful
  for development, especially for understanding what's required in terms
  of the API.
* More extensive debugging outputs for coadd assembly and background
  reference construction.
stack: Make debugging outputs optional

Merge from two slightly different lines of development.
backgroundModels: clean up and optimise

Big speed-up (~9x) on calling PolygonBackground.fromImage()
and .getImage().
stack: clean-ups
backgroundModels: fix PolygonBackground.merge()
backgroundModels: add more extensive extrapolation options