Add SLITWIDTH to slicecam headers

[chazshapiro]

SLITWIDTH can be in the primary since it's the same for L and R

The WCS coordinates will depend on SLITWIDTH because the image moves as we open and close the slit.

I think putonslit should still work because it is just counting number of pixels to move and multiplying by CDELT, not using absolute coordinates.

[astronomerdave]

implemented in commit 01100b1922f73fb1914a905aa440d5ddabbf60ef