COMP: Enable MultiVolumeImporter only if Numpy support is enabled

Since the module depends on numpy, excluding the module will avoid
startup error when the module is loaded when numpy support is disabled.

A better approach could consist in teaching the module how to detect if
numpy is available and fail graciously by reporting a warning. A issue has
been reported upstream. See https://github.com/fedorov/MultiVolumeImporter/issues/6

Co-authored-by: Chrystel Juan <chrystel.juan@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22144 3bd1e089-480b-0410-8dfb-8563597acbee