Store offset values in gen2 MEMORY status dict

When any gen2 script makes an offset, that should be recorded in some MEMORY slots. These will be relayed into the PFS system and recorded at the start of each AG/SPS exposure. Among other things, this means that the MEMORY values should be cleared when the actual offsets are cleared by slewing, etc.

The "dither offsets" are by far the most important.  Rotator and focus "guide offsets" are also desirable, but any other applied moves (az and alt) should be stored if possible.

All keys should go into the MEMORY.PFS table space. Under that, I think we can just use a flat namespace, with DITHER_RA and DITHER_DEC, etc. for the dithers, and AG_IR, AG_FOCUS, etc. for guide offsets.

If we ever expect to make user offsets for other reasons (i.e. not just dithering), I propose we use USEROFFSET_RA, etc. instead of DITHER_RA.