Enhancement: implement frames for exposure

[ddkohler]

Currenlty, acquisitions consist of a single exposure. Cam allows one to configure frames (e.g. FrameCount), so that a single acquisition is composed of multiple separate images. It is useful to break up long exposures into many acquisitions (e.g. cosmic ray filtering).

Not sure if frames should be hidden from the client (so that always get one image per acquisition), or if they should expect an frame every x seconds of exposure and can do with each frame as they please. Perhaps that should be a configurable option.