gui: GUI update
pvcam: automatically activate shutter when needed

Shutter is useful to avoid smearing (which can happen on the SPARC).
So activate it, but with parsimony as its lifetime is quite limited.
cli: fix reading sensor metadata

typo
Merge branch 'master' of vpn.delmic.com:odemis
doc: add warning when connecting the Pixis camera via USB hubs
cli: space cleanup

no behaviour change
model: improve dataflow proxy ending

__del__ is hell
the thread might run while in a __del__ state
the value of zmq's constants change from an int to None during __del__ !

Try to workaround all this crazyness.

Main result is that there is no more exceptions and blocking when stopping the cli.