Temporarily hardcode custom HGTD materials
This MR addresses the crash reported in ATLASSIM-5301 when running tests with upgrade detector configuration. Many materials that were being used when building the HGTD geometry have recently been (or soon will be) removed. As a temporary (i.e. pre-GeoModelXML) workaround, these custom materials are hard-coded into the building of the HGTD.
Edited by David Richard Shope