Added an "add_cursor_line" function to the isisrc_line_models script

that allows for adding line functions via the cursor function in the
pgplot routines.  It's not 100% yet (the "subtract" qualifier isn't
being passed through and parsed properly; *** however ***, I do plan
on some backwards incompatible changes to the "subtract" qualifier -
hence the 0.4 version number on this).

These routines should be considered to still be in the "design" stage,
so high level design comments definitely appreciated, and be aware
that details will change as they develop.