Remove redundant HGTD namespaces
This MR touches many files, but the changes can be summarised as the removal of 3 namespaces for HGTD which are all redundant due to HGTD already appearing in the names in question: HGTDet was being used for HGTD tracking geometry, HGTDGeo for the HGTD detector factory, and HGTD for the HGTD clusters. Equivalent removal of namespaces also to follow soon in 21.9.
Edited by David Richard Shope