Reworked channel changing: now it tracks the current ch. and resets only once. This makes the program to finish faster.
Added rolltime_down and percentage values to shutter structure: now the schedule file needs to contain a percentage column after the timestamp for lowering, and a rolltime needs to be defined for each shutter in the beginning of the program. When lowering shutters, there's a calculation to sleep until the desired percentage is reached, when the shutter is stopped
The way to deactivate shutters has changed: now you need to set the percentage to 0.
