Merge branch '21.0-FastCaloSim-dev' into '21.0'

21.0-FastCaloSim: Changed coordinates used for shower shapes simulation

See merge request atlas/athena!16320

(cherry picked from commit 02984203bb2d4b6d6178d4f99eee550273ffdc37)

b43eade7 Added HepPDT to CMakeLists
d78c75e0 Implemented sign flips for negative particle charge and eta
3686c1bc Changing to new coordinates used for shower shape parameterization in FCal
7f9bb2fa Added comments to the code to explain changes in signs in shower shape coordinates