[evlatests] PDif effectiveness at C-band

[Rick Perley]

    Same story as at the high frequency bands.  Applying the PDif values 
removes *all* short-term variations in the visibility amplitudes which 
are not clearly induced by pointing errors.  All PDif values are of 
suitable value and quality. 

    Most long-term trends are also removed or reduced -- but others are 
introduced.  It's clear that the remaining trends in the visibility 
amplitudes as a function of elevation (or time, since I was observing a 
single source) are electronic in nature.   The amplitude of these 
residual trends is small (couple of percent, typically).  But whether 
these are due to elevation or temperature cannot yet be determined with 
confidence. 

    All Tant values from on/off 3C84 are close to the expected values.  
All Tsys values are also reasonable. 
   
    Nothing but good news!