[evlatests] new file for focus checks

[Ken Sowinski]

/home/mchost/evla/scripts/operations/focuscheck.evla

is a new evla script which can be used to cause telcal to estimate
focus offsets and record the results in its usual repository, just
as it does with pointing and delay measurements.  As written, the
script makes a pointing measurement at X band which is used to
corredct the pointing for a succession of pairs of focus measurements
at Q, Ka, K, X and C bands.  The file may be run at any LST and will
choose a suitable source; it will stop normally after all the
measurements are made which should take about half an hour.  I suggest
that we begin using this to verify focus settings during startup each
week.  If we gain sufficient confidence in the antennas (and the solver)
we may reduce this to once a month.

This script does not measure Ku band offsets; I don't see the need
for the dwindling number of VLA antennas.  This can be added when
we start to oufut EVLA antennas with Ku band antennas.  Nor does
it measure L band focus offsets because that is probably best done
with an experiment crafted to that purpose.  It might be interesting
to put together a script to measure focus offsets at the edge
frequencies of one of the wideband feeds.

If necessary, an analog to 'delayup' will be written to digest the
results and create an update file for parminator.  On the basis
that the focus settings ought to be constant my inclination is to
not make it too easy to change the values in the database.  Many
years ago Mike Kestevan bequeathed to us a program to solve for
focus curves given a set of focus offset measurements over a range
of elevations.  Eventually I will make sure that it can digest the
file produced by telcal.