acq: break stream into sub modules

should not change behaviour
acq test: support running the backend from pydev

we keep getting "Import by filename is not supported."
if that trick really works, we could use it for all test cases.
stream: fix doubled line
acq: add RGBCameraStream

Allows to handle RGB data, and directly pass it.

Also introduce ensureXYC() to swap dimensions if needed.

Untested... so need to introduce test cases.