scan stops during fly scan with a camera time out error

[decarlof]

The following condition:

• Flat exposure time is set to be different from projection exposure time
• Flat field is taken at the beginning of the scan
• Projection exposure time is much longer than the flat exposure time (2s vs 0.2s)
• The scan has a large number of projections (>3000)

exposes a bug that causes the scan to stop during fly scan with a camera to raise a CameraTimeOut error.

The main issue is that the function set_exposure_time() runs only when the scan is not running but it is also used by functions that are called during the scan with no effect. #144 addresses this issue.