Fixed vtkImageGradient in parallel.

vtkImageGradient did not work in parallel because it assumed that
the input extent is the same as the update extent, which is not
the case when there are multiple pieces requested. This bug is
prevalent among the imaging filters but for now I am fixing them
one at a time. Fixing them all at once is non-trivial.

Change-Id: I28bdd56e46c97ef7a80e18a46b6d266fbcd79134
Add support for EDGEFLAG attributes in legacy reader & writer.

So far edgeflag attributes were not serialized/unserialized.
This miss prevent edgeflags to be transfered in ParaView
data delivery mechanism - and avoid edges to be correctly
represented by the client in client/server mode.

Change-Id: I64d6c06f8fa6d1c92f9156d973b28c863b9efa1f
bugfix 14459: Iterating through polydata cells incorrectly.

RemoveDeletedCells() and RemoveGhostCells() was not
iterating through the cells correctly and so cell
data was not getting properly updated in the resulting
polydata. This fixes that iteration order. Also added
a test to show incorrect behavior and that it is now fixed.

Change-Id: I2654618c38dc243272a80daa99f56f63082c7a70
Fixed issue with ghost cells not being deleted properly.

Previous commit for issue 14459 exited out of RemoveGhostCells()
prematurely and thus didn't properly delete the requested ghost
cells under certain circumstances. This fixes that bug.
Also improved some tests to make them more analytical, give
better feedback on failures and build issue on Windows.

Change-Id: If44eaadf9774dc1606f9cc581028719d8c6f80ab
Merge topic 'add_edgeflags_io_support' into master

8efe5ef2 Add support for EDGEFLAG attributes in legacy reader & writer.
Removing temporary file that accidentally got added.

Change-Id: I72a95525282b9e1599e7bd72ab1f5a8e87390056
Merge topic 'image-gradient-bug-14982' into master

869c2d1a Fixed vtkImageGradient in parallel.
Merge topic '0014459-RemoveDeletedCells-3' into master

4615e591 Removing temporary file that accidentally got added.
9cd28a60 Fixed issue with ghost cells not being deleted properly.
44131c6d bugfix 14459: Iterating through polydata cells incorrectly.