add --readout-mode=N option

sbig-info ccd lists a camera's variable number of binning modes like this:

mode-0:             320 x 240  1.25 e-/ADU 10.00 x 10.00 microns
mode-1:             160 x 120  1.25 e-/ADU 20.00 x 20.00 microns

but sbig focus --resolution and sbig snap --resolution accept only "hi", "med", and "lo".

It would be more flexible and probably clearer if modes could be specified by number, as output by the info command.

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