Clean up how framerate is handled

[jmtayloruk]

Framerate is handled in several separate ways:
self.settings["brightfield_framerate"] (which is used in optical_gater_server etc, but does not set seem to be set in the .json file?)
self.settings["framerate"] (which is read in prospective_optical_gating.py, but not used elsewhere)
self.framerate (which seems to be redundant, though I'm not 100% sure it doesn't have some relevance to the Pi - though there is also camera.framerate on the Pi)

This should be consolidated - I think they are all referring to the same thing.

[ChasNelson1990]

• camera.framerate is needed for the pi
• self.settings["brightfield_framerate"] does come from the settings json? It's definitely in the example json.
• self.settings["framerate"] is, I think the old POG name for "brightfield_framerate" and so let's update this to be "brightfield_framerate"
• self.framerate.... sounds like it was there when OpticalGater inherited from PiArray... so, yes, redundant now