add optional visit argument for ccdActor read command

We still don't have the gen2simulator up and running at LAM, so I've worked on a very simple actor (ics_seqnoActor) which generate the visitId as a keyword.

As a result, when the sequencer(ics_spsaitActor) is asked to take an exposure, it's getting the new visitId from the seqnoActor and parses it as an argument to the ccdActor(s) read command. 

typical read command from spsaitActor look like that :

'ccd_r1 read %s visit=%i exptime=%.3f darktime=%.3f obstime=%s' % (exptype,visit,exptime,darktime,dateobs