[evlatests] VHF support

[Ken Sowinski]

Changes have been made to make P band and 4 meters more likely
to work.  The executor changes will be available at the next
update.

1.  IFs A and C are ALCed in software just as at the other bands.
2.  The FIR gain parameter is reduced from the default 30 to 15 at
     P band and 5 at 4 band to keep the T5 from being overdriven.

Some of the BD attenuators will have to be adjusted a little to
accomodate this.  Experience will tell us whether further adjustments
are necessary.  A side effect is that the FIR gain parameters are now
accessible from a script so that systematic studies of performance
as a function of downconverter attenuator and FIR gain parameters can
be made entirely under script control.

There have been changes in the Modcomp Free world as well.
1.  The VLA antennas are now setup more correctly for P band and
     stand a chance of working.
2.  Psysstart.evla and 4sysstart.evla now set the various BW
     parameters as expected for the VHF bands.  These file are
     in the usual place: /home/mchost/evla/scripts/operations.
     Both files have been setup to choose the same sources that
     the corresponding VLA file would choose to make working with
     the Modcomps easily possible.