Import HGTD GeoModel from 20.20
This MR brings in the HGTD GeoModel from 20.20 as a first step in getting the HGTD up and running in 21.9, creating a new package structure for HGTD according to ATLSWUPGR-66.
A couple of slides were shown recently in the first of a series of meetings covering 21.9 HGTD integration.
Edited by David Richard Shope