Refactor GetTemplateTask to merge then warp

Significant refactor of all parts of GetTemplateTask, to achieve a ~2-4x
speedup when using cell-based coadds.
* Change `run` input to take a dict of tract_id->list(exposures), so that we
can merge the tracts, warp them, then merge the warps.
* Move schema definition to init.
* Make exposure catalog in separate function.
* Cleanup docstrings and comments.
* Rework the test for the new API and add more tests and a more useful
debug plotting framework.