Make it easier/possible to plug in a user-defined column parsing function (e.g. for lddate)

[jkmacc-LANL]

Some columns like lddate don't have a well-defined string format. Following this question from @iceseismic on the user's email list, it would be very useful for users to be able to easily plug in their own parsing function for a given column. Maybe a .pisces.cfg file with an lddate_format variable could be a solution.