Add option VTK_DATA_STORE to specify ExternalData object store

Replace the ExternalData_OBJECT_STORES cache entry with a VTK-specific
name "VTK_DATA_STORE".  In the common use case there is only one store
so make the option a "PATH" cache entry for easy handling in cmake-gui.
In order to allow advanced users to configure multiple stores honor any
ExternalData_OBJECT_STORES that is set but do not add the cache option
by default.

Select a default for VTK_DATA_STORE based on the results of local user
configuration.  First, teach the SetupForDevelopment.sh script to run a
SetupExternalData.sh script that prompts the developer for a default
value.  Second, if no default was selected by developer setup, look for
a VTKExternalData or ExternalData directory as a sibling to the source
tree.  Third, if the previous two attempts failed, fall back to the
environment variable ExternalData_OBJECT_STORES for advanced users.

Change-Id: I964cb99aa204176010f27948a9bd40a00836532f
Merge topic 'ExternalData-default-store' into master

d7bf9e9a Add option VTK_DATA_STORE to specify ExternalData object store