CryoScope 2.0

[astronomerdave]

This implements CryoScope as the first customer of the new camerad-2.0.

[astronomerdave]

I'm starting this as a draft PR until it's finished. I've only added the producer thread. I'll add the consumer thread next.

[astronomerdave]

Cryoscope is not fully functional here yet, but this adds things of general interest. A brief summary follows:

1. organizational change: moves Camera::ExposureModeRXRV (and support functions related to ExposureModeRXRV) out of archon_exposure_modes.cpp and into its own file, archon_exposure_modes_rxrv.cpp, because once we start adding code to all of the exposure modes, putting them all into the one file would make it huge.
2. new code: adds functionality to Camera::ExposureMode_VIDEORXR::image_acquisition_thread that wasn't there before
3. new code: passes exposure mode arguments when constructing the exposure mode object in Camera::ArchonInterface::set_exposure_mode
4. new feature: implements a proper FIFO wrapper to the enqueue object that the producer/consumer threads use. This is in its own file, image_queue.h
5. new feature: adds new functions to Config class to get optional and required config keys.