[fitsbits] CRPIX clarifcation

[Jonathan McDowell]

> Bill - good idea, but shouldn't the last sentence be:
>   Note that the
>      reference point may lie outside the image and that the
>   CENTER OF THE first pixel
>      in the image has pixel coordinates $(1.0, 1.0, \ldots)$.
> 
- since we have talked above about floating point pixel coordinates
and the implication that they define an interpolated coordinate system,
we should make sure that this interpolation is unambiguous.
Tradition has been that the WCS corresponds to the center of the pixel
on the sky, I believe.

 - Jonathan