[daip] keyword for aberration correction

[Walter Brisken]

Today at the VLBA Test and Development meeting I mentioned that with the 
software correlator I plan to use UVW coordinates that are corrected for 
the various relativistic effects (aberration, etc).  I believe this should 
be made clear in a table keyword.  I would like the keyword (call it 
ABERCORR here, subject to other suggestions) to have at least three 
levels:

ABERCORR = UNCORRECTED -- the uvw values are those returned by calc
ABERCORR = APPROXIMATE -- a correction, acturate to some level, is made
ABERCORR = EXACT -- all corrections consistent with the delay model are made

Does this sound reasonable?  I would suggest making this an optional 
keyword in the UV_DATA table of FITS-IDI files.  The default if left 
unspecified would be UNCORRECTED.

-Walter