BUG #13799. Variables not showing curretly for Comparative View.

The problem was that the representations in each view were being updated using
repr->Update() instead of simply letting the view update the representations as
needed. Since vtkCompositeRepresentation::Update() no longer forwards the update
request to internal representations, it meant that the UI ended up updating
using old state of the representations -- hence the missing variables. This
fixes that issue.

Change-Id: Ia25e7dd8f4d5c0c368692894003567eb8bf79043
Fixed Overlay mode in Comparative View.

When in Overlay mode, all representations were simply showing the first
timestep. This is because when the view was updated for (0, 0), it would update
all representations in it, including those for other locations in the
comparative grid. The caches for all those representations would be filled and
subsequence updates would  have no effect. This addresses that issue by clearing
caches for representations at (i, j) location just before updating the
representations at (i, j) location.

Added a test for the same (ComparativeOverlay).

Change-Id: I05493e4258e54a7dc9b4554c56f40f8271e22f21
Merge topic '13799_comparative_view_fixes' into next

87e9016 Fixed Overlay mode in Comparative View.
70f9e29 BUG #13799. Variables not showing curretly for Comparative View.