[evlatests] More on noise for mode 'PA'

[rperley at nrao.edu]

George has confirmed that the factor-of-two increase in noise is seen,
using CASA, on 'PA' mode data taken at 20 cm on 3C147. He also confirms
that the effect is absent in 50 MHz continuum data.

I then remembered that I had taken data in mode 'PA' for observations of
3C273, back in 1997, with the same BW (12.5 MHz) as the recent tests.  I
have loaded these data, and did a quick calibration.  The result of this
is that there is *no* difference in the noise, as shown by the histrogram
distributions in real and imaginary parts, of the four correlations.

Thus, it appears that this problem is a relatively recent development.