Fix wrong PV names configured for quadrupole elements

A bug in load_mml was causing quadrupole elements to be configured with
the wrong PV names. This meant that when doing tunefb, we were writing
to the wrong PVs and wrong elements, causing tunefb to go wrong. I have
changed load_mml so that it looks up the element indexes in the
quadrupole family list of pvs, rather than in the combined quadrupole
list of pvs.

Author: ptsOSL