Camera base extension

PR adding functionality to the base `Camera` class. (#148)

- [x] make `grab` in such a way that it _always_ returns a frame, no matter what the `trigger_mode`
- [x] replace the `acquire` with something better
- [x] refactor dummy cameras to use some `DummyBase` to reduce repetition
- [x] make the trigger mode visible in the parameters table
- [x] get rid of GError
- [x] refactor uca data taking
- ~~[ ] use `start_recording` in the context manager~~

_EDIT:_ The last point is skipped becaues the `__enter__` doesn't take any arguments and we cannot assume the user wants to start recording all the time, e.g. by a `BufferedCamera` one might want to readout the buffer inside the `with` block.