driver semcomedi: try harder for finding the best dwell time

Use both types of searches, and pick the best result.
Still need to be tested on real hardware.
driver simsem: fix voltage values

It's kV, not MV (typically)
model: avoid expcetions when unregistring VAs

adapt code from dataflow. This avoid some exceptions when closing Odemis.
gui: add TODO comments about the menu

Open odemis-develop.pdf as developper manual.

various potential improvements to the menu
gui main: only attach logger to GUI once it's initialised

otherwise, error during GUI initialisation are not displayed on the console
(only visible in the log file)
gui main: very minimum to support the "secommini"

secommini name might change, but we need something right now.
install: first version of fake secommini

based on the simulators

chamber is quite different from the real secommini chamber (no overview position...)
overview ccd return greyscale 16 bits instead of RGB.