util: add comment

we need to simplify the meaning of the correction metadata
acq stream: new AlignedSEMStream

Special stream for the Delphi that does like a SEM stream, but will automatically
run a calibration whenever it's played at a new stage position.

calibration function needs to be moved and still needs more testing.
gui streams: special SEMStream for delphi

use our brand new AlignedSEMStream.

It's not fantastic, because:
 * the GUI will block until the calibration is done, which can take up to 4s
 * with the simulator, the calibration always fails as it can find spot in the CCD image
 * when the calibration fails, the error message is displayed in the debug panel, instead of a small pop up