Merge topic 'HTG_GCG_GetIndexFromLvl'

4ea563a3 Update HTG GCG to use the right method to retrieve neighbor tree

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Reviewed-by: Loic Gaillard <loic.gaillard@kitware.com>
Reviewed-by: Louis Gombert <louis.gombert@kitware.com>
Merge-request: !10881