Finalize exposure time sources and keywords

We have a proposed mechanism for establishing the exposure time and duration values, but need to make all the actors agree.

• the ENU actors (specifically the shutter control) establish the absolute exposure start time (based on some point in the shutter opening process) and the exposure length (based on the shutter closing and the previously decided open time).
• the CCD actor read command will be passed those as "exptime=S.SS" and "dateobs=string".

ISO format, UTC.

Questions:

• What FITS keys to generate?
• what to do about bias and dark readouts? I think that the dateobs argument should be optional and the exptime argument disallowed or ignored.