Xspress3Channel class

[bruceravel]

I wish that the Xspress3Channel class in nslsii/detectors/xspress3.py defined an attribute like this near line 433

extra_rois_enabled = Cpt(EpicsSignal, 'PluginControlValExtraROI')

Seems silly to subclass just to add this obviously useful attribute.

[mrakitin]

PRs are always welcome! Is it a standard part of the detector?

[mrakitin]

We discussed it with Bruce today, and decided to give it a try at a couple of beamlines which use xspress3 (QAS and SRX maybe) to make sure it does not break what they are doing.