VP1: Add support for loading config files (missing since CA move)
Also:
- Make Filename optional (this is still not perfect, since we used to be able to pass multiple files). This also meant I had to move
HLTP
check to after empty filename check, otherwise the autoconf kicks in and fails. - Fix AtlasExtrapolation error reported in ATLASVPONE-659
- Misc minor fixes
cc @rbianchi
Edited by Edward Moyse