acq stream: add support for width in 0d spectrum

compute the mean of the pixels which are situated < width/2

Tested a bit, but will need more testing, on actual data.
gui settings: make sure VA choices contains the current value

This is obviously the case when we get choices from the VA.choices, but if gotten from
the settings, then we might end up with choices not containing the current value, which
is surprising to the user.

=> do the same as for the range
Minor change
Merge branch 'master' of delmic-server.local:odemis