Calibration fix

There was a serious issue with `LinearCalibration`, constituting in the fact that if the units are e.g. meter and a user specifies a value in mm, then the `to_step` would return something like `x q.mm / q.m`, which does not get automatically converted to base units when e.g. passed to a device in `Connection.send` method. The [test](https://github.com/ufo-kit/concert/blob/calibration-fix/concert/tests/unit/test_calibration.py#L23-28) addresses this issue.
Another problem was the module where calibration resided, not only motors might want to be calibrated. For now it's under `concert.devices` because I couln't come up with anything what is not a device that would want to be calibrated.