Added test for coloring multiblock data set by field data. The
multiblock data set consists of vtkImageData leaves. It was failing
when running in client/server mode with the server run via mpirun with
two or more processors.

The problem was located in
vtkMultiProcessControllerHelper::MergePieces.  Enabling copying of
field data in the vtkAppendCompositeDataLeaves filter used to merge
leaves in multiblock data sets fixes the problem exposed by the test.