Distribution: Fixed interface to DMPlexDistribute()
Fixes to comments.
Updated Distributor Python object for removal of partitioner parameter.

The 'partitioner' parameter is now a PETSc setting.
Update Distributor for moving partitioner to PETSc setting.
Fixed partitioner settings.
Use PETSc partitioner setting.
Merge branch 'knepley/upgrade-petsc-master'

* knepley/upgrade-petsc-master:
  Use PETSc partitioner setting.
  Fixed partitioner settings.
  Update Distributor for moving partitioner to PETSc setting.
  Updated Distributor Python object for removal of partitioner parameter.
  Fixes to comments.
  Distribution: Fixed interface to DMPlexDistribute()