Flush out definition/use of channels

[adamkglaser]

At the moment channels are only defined by cameras, lasers, and filters. We should expand the definition of a channel to be the entire microscope and any device that is involved in that particular 'channel'. Including stages, etc.

This can then be used to simplify the UI, where a drop down menu allows selecting a channel to use, which then updates all devices in the UI based on the devices that are defined for the given channel. At the moment, it is confusing to have the radio button for the channels, while also showing all laser sliders (all but one of which are not used for the checked off channel). This would also reduce the clutter of the drop down menu that lists all of each particular type of device. Having these lists is nice/generalized, but we can assume that the user will define each channel properly and there is no need to mix/match devices in real-time with the GUI by using the drop down menus to switch devices.

We should also decide where the channel definitions live. Right now it seems to be duplicated in the GUI yaml and instrument yaml. I suggest we have a dedicated channel yaml.