[daip] negative ZINC

[Gustaaf van Moorsel]

Following a discussion with Eric on this subject:

For publication style plots of spectral line data, it is often
desirable to have velocity run from low to high.  Since in AIPS
spectra tend to run from low to high frequency (and therefore
from high to low velocity), it would be advantageous if plotting
programs would allow the user the choice to invert the direction
of the third axis.

I noticed this first in KNTR, where the obvious adverb, ZINC, does
not allow negative values.  As a work-around, Eric suggested to me
I use TRANS with TRANSCOD='12-3' (an option I was unaware of until
then) first.  Though this solved the problem, running KNTR on the
original cube with a negative ZINC would have been preferable.

Later I noticed to my pleasant surprise that ISPEC does allow ZINC
= -1, with the desired functionality.  I would therefore suggest
KNTR (and possibly other tasks where inverting the velocity axis
makes sense) be modified to allow a negative ZINC as well.

Thanks!

Gustaaf