Bin by pixel

[jokasimr]

Addresses part of #217 by binning the McStas data by "pixel" after loading and adapting the workflow to handle the pixel-binned data structure.

[jokasimr]

Questions that remain answering

• what is the number of pixels in the detectors should be divided into?
• probably we should load the detectors separately, with a type parameter determining which detector to load, like in other instruments with multiple detectors