[daip] FITLD feature request

[Walter Brisken]

This is a trivial feature request, I think.  I would like FITLD to turn on 
all VLBA corrections for all data correlated with the DiFX software 
correlator.  Rather than overwrite the TELESCOP name (currently VLBA, 
which is the field used to enable these corrections), I've introduced a 
new keyword called DIFX which has as a value the version number of the 
correlator.

It might be that programs other than FITLD need to pay attention to this, 
for example BPASS certainly does too.  Maybe the question I should have 
started with is "What's the best way to enforce VLBA-like behavior in AIPS 
for any DIFX correlated data".  I'll end with that question instead.

-Walter