pvcam: ensure that temperature update is never done during acquisition

never noticed any problem, but was told that it can be a problem

Also ensure that we wait long enough for the readout to finish
cli: fix set-attr with EnumaratedFloat VA

float + choices were often impossible to change because of the rounding error in floats.
Pick the closest value from the allowed one, to help the user

also indicate the min/max value of an image, that might help to see what changes
driver: add new spectrometer component

A CompositedSpectrometer component, which can take a spectrograph and a DigitalCamera to
make a complete spectrometer.
so far, only the basics, not working at all.