User hook for command FIND

The CLASS command FIND allows the user to make a custom selection from the Input indeX (IX: input file summary), building then the Current indeX (CX). This is done through the option /USER. If CLASS knows about a User Subsection owner and title, then a hook can be executed in order to make a custom search depending on this User Subsection contents. The basic steps to be done by the Subsection owner are:

1. tell CLASS which subsection it owns with class_user_owner. This is optional if it has been done elsewhere.
2. declare to CLASS with class_user_find a subroutine which will parse the FIND /USER arguments,
3. declare to CLASS with class_user_fix a subroutine which will make the selection depending on the command line arguments and the Subsection contents.