Fix problem reading buffer from matplotlib

matplotlib.backends.backend_agg.FigureCanvasAgg.print_to_buffer()
prints an image to an old-style Python buffer object and ParaView was
using the API for the new-style Python buffer object. Serendipitously,
this works in Python 2.7.5 (on my development machine), but that is
not so on other Python versions. Switched to using the API for the
old-style buffer object and verified that it works with Python 2.7.2.

Change-Id: Idfa91c27109756e275d690229865e5f648249bda
Merge topic '14491-fix-matplotlib-buffer-object-access' into next

505bff5 Fix problem reading buffer from matplotlib
Fix tests failing on non-mpi builds.

These tests need MPI enabled since they use D3. Fixed that. They were failing
since D3 is no longer available in non-mpi builds.

Change-Id: I5718d076858fed26d6c6a41b8c6594756cdc95f2
Merge topic '14488-fix-mpi-dependent-proxies' into next

36efd24 Fix tests failing on non-mpi builds.