acq stream: fix auto bc on static AR stream
acq stream: rename ._irange to ._drange

This is the data range.
The main goal is to avoid confusion with the intensityRange, sometimes named irange,
which is the visible data range when doing a projection to 8 bits.
gui: ensure that empty data can be "seen"

don't fail if drange = (0, 0)
report the edges of the histogram as (0, 0), instead of (-0.5, 0.5)
driver simsem: extend simulator to provide new VAs

Should allow to simulate new fully controlled SEM.