Reduce the number of restarts of timepix device on initialization

[troehling]

When launching pymepix-viewer a TimepixDevice object is created. This object can be started and stopped (the associated pipeline is started and stopped). Some operations (like changing the number of processes used in the pipeline) require this to be restarted. Some operations might not require this restart.
On initialisation of the application multiple operations are implemented which do currently execute a restart of the timepix device. Overall the device is restarted three times (if I counted correctly) although only one start would be required.

The objective is to:

• Eliminate all stops from __init__method to optimise the loading time of the application
• Eliminate restarts (start and stop) from all operations where it is not required. Right now it is not clear which operations do qualify for this. In the worst case (which I do not hope) the restart is required every time.