Don't replace an algorithm's executive if a proper one already exists.

In vtkSISourceProxy, an algorithm's executive is automatically
replaced with a given executive. Reasons to avoid this:

1) if it is the same executive, we have unneeded computation

2) if it is an executive that is derived from the requested
executive it replaces the desired one with a superclass of
it. For algorithms with specialized executives that are set
as the default executive for them it will replace them.

This fixes the issue.

Change-Id: I361e552d822fcfea43bdfc90a2929c1f509fa0fc
Merge topic '13973_do_not_auto_replace_algorithm_executive' into next

57bb7aa Don't replace an algorithm's executive if a proper one already exists.