FIVPT/ONOFF don't handle DBBC IF detectors correctly for continuous cal

[wehimwich]

@jfhquick reported that fivpt and onoff don't handle the DBBC(2) IF detectors correctly for continuous cal.

The DBBC(2) IF detectors don't provide separate cal-on and cal-off power for continuous cal (only total power is available). Supporting them in an ad hoc way for this case was never completed. As a result, they did not work for pointing with fivpt and the Tsys in onoff was oddly valued. They can be made more useful by (internally) changing their Tcal for this case to -100 (i.e., no cal at all). The result should be that the (net) Tant comes out in precent of Tsys in fivpt, making it possible to find a peak, and -100 is displayed for Tsys in onoff. Curiously, the help page for fivept described using this method, but it was not implemented.

Getting useful results for these IF detectors depends on having accurate values for the "DBBC IF detector factors" in equip.ctl.