This vertex generator works like the VertexFile_Gen, but instead of
reading RAT ROOT files with DS::MC information, it creates MC primaries
from the StdHep fields in a GENIE RooTracker file. (This avoids the need
to compile RAT and GENIE in the same environment, and to do a RAT-specific
conversion step (genie2rat); rather, convert GHEP using GENIE's
`gntpc`.)