`PstFrom` use of underscores in auto names

[briochh]

I'm thinking of dropping some of the underscores in the automatic par and obs names in PstFrom() to make extraction of metadata within names easier. So instead of "use_col:x_..." we would have "usecol:x_..." .
Super minor but just checking for anything unforeseen.

[jtwhite79]

I think anything we can do to make parsing those names into metadata is a win...could even have a helper function to do it for us! Maybe in pst load?