semcomedi: use official numpy way to get the maximum value of a dtype

clean up only
Increased GUI log file size to 1 MiB
This commit prevents the custom Slider controls from firing an 'unlimited'
amount of events in a short time. They will now fire once every 50 ms at
the most.

This limitation also seems to have resolved a bug which was causing an
infinite loop with Sliders that were connected to VigilantAttributes. This
was not caused by float type inaccuracies, which were encountered earlier
and have been resolved. What the actual cause was, is not fully clear, but
the control would start going back and forth between two values, e.g.
0.157049655914 and 0.151920482516.
Allow for a bigger error margin when comparing float values with Slider widgets
img: add function to compute optimal brightness and contrast

this will allow to smooth the transition between automatic and manual b/c

also fix corner case for B/C computation, to ensure automatic and manual b/c are
really identical
Slider still not stable, can lock up if there's two active slider connected to the same VigilanAttribute
gui: use previous b/c values when switiching to manual b/c

the current values are computed and set.
However, for now the widgets are not updated, so it looks pretty weird.
Reverted changes to the wrapper functions, because they were causing infinite recursion
Merge branch 'master' of vpn.delmic.com:odemis
semcomedi: log exceptions in the acquisition thread
gui: fix optimal bc function