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.