Add datacube and an atomic guiding ROI keyword

[mikelangmayr]

• Add datacube, so a frame sequence can be written as one FITS cube. It is generic to any camerad camera, so it sits with the other generic keywords
• Add guidingroi, which takes all four bounds as "y0 y1 x0 x1" and applies them in one step. Writing the four individual keywords re-derives and re-applies the whole camera mode each time, which is four detector reconfigurations and three intermediate geometries; this is one. The individual keywords stay, for reading and for nudging a single edge
• Reject a malformed or non-increasing ROI before anything reaches the controller
• Verified against the Archon emulator: datacube round-trips, guidingroi reads back the bounds in force and sets all four at once, and the validators reject bad arity and bad ordering. Four individual writes took 0.036s against 0.007s for one guidingroi. pylint 10.00/10