pvcam: support synchronisation

works kind of fine, with different exposure time.

The main problem is that the dwell time must be computed from the exposure time, with a formula pretty hard to guess

Also problems sometimes with the pvcam driver after the syncrhonised acquisition is used (timeout)
doc: improve some install commands
driver: improve test
config: add spectrometer to sparc config
driver: add children to the spectrometer component
model: share events remotely also as Event, so that they are easy to list