Merlin: add some useful parameters to the controller

[sk1p]

For "active" mode, i.e. where we take full control of the detector, we have some useful parameters we set when using DECTRIS detectors. This is not yet implemented for Merlin, and means it can be quite complicated to configure the detector (see run-udf-on-merlin-data for an example...). The user currently also needs to take care that they don't mess up the command order, instead of us making sure of it once.