[fitswcs] Long-slit spectroscopy WCS

[Frank Valdes]

An addendum to my last posting.

> 
> w = CD1_1 (x - CRPIX1) + CD1_2 (y - CRPIX2)
> s1 = CD2_1 (x - CRPIX1) + CD2_2 (y - CRPIX2)
> s2 = 0
> 
> The WCS is still 3D but in effect we set CRPIX3=1, CDj_3=0 and CD3_i=0.

To avoid an ill-defined matrix what you actually do is set CRPIX3 to 1
and CD3_3 to a non-zero value, corresponding to the scale of deg/pix:

w = CD1_1 (x - CRPIX1) + CD1_2 (y - CRPIX2)
s1 = CD2_1 (x - CRPIX1) + CD2_2 (y - CRPIX2)
s2 = CD3_3 (z - CRPIX3)

Frank