wip
gui: Fixed calibration file loading (Assigning calibration data still causes errors)
Merge branch 'rinze_calibration'
Fixed merge conflict in acq.stream
util: add a Subtract function

numpy has almost what we need, but underflow/overflows are really annoying, so avoid them.
acq stream: fixes for calibration loading

no need to raise nicely exceptions, the calib data should have already been checked by acq.calibration

subtracting images is not that easy

fix comparison of AR shape

don't complain when applying calib to spectrum without wavelength info

Kludge to also update the 0D spectrum when the spectrum calibration changes.
gui: various fixes for the calibration files handling

Still some minor problems when the loading fails, but otherwise it should be pretty fine.
gui: fix when changing spectrum calib file and not point is selected