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