EVR channel object, top level object, custom Screen

[slactjohnson]

Current Behavior

The EVR Trigger class has many of the PVs that are useful when working with an EVR channel, but doesn't have all of them.

The Typhos screen generated by this class is... interesting.

Expected Behavior

For example, the :CNT can be nice for debugging, and in some cases we may need the CTRL.IPOE PV (release/acquire). As noted in #515 this object is missing the ticks PV.

Additionally, it would be good for us to have some custom screens, similar to those that we have for the EVR in EDM.

Once we have a good trigger class, it should be easy to roll a bunch together with a few additional PVs to make a "full" EVR object and screen.

Context / environment

I'm looking to add an EVR channel to a device (a spectrometer) but I anticipate there will be more of this in the future (cameras, etc).