Change slit offset FITS cards

The simulator used what seemed like a decent name for the slit offset, used by the dithered flat code. But we now understand the slit hexapod better, and will use more appropriate terms. Both cards use microns for slit shift.

Specifically, we have existing simulated files with sim.slit.xoffset. The files from LAM have W_FCA_SHIFT.

I am hoping that the pipeline can recognize those two different card names in the short term. And quickly, since we can get real dithered flats, perhaps today..

To be complete, the offsets are:

• W_FCA_SHIFT
• W_FCA_DITHER
• W_FCA_FOCUS

and the rotations are:

• W_FCA_PITCH
• W_FCA_ROLL
• W_FCA_YAW