Refactor pixel size
This PR is a refactoring to simplify the TomoDetector
class.
Previously the sample pixel size was computed inside the class.
Now we provides 2 dedicated pixel size geometries which are outside and can be used as helper.
UserSamplePixelSize
AutoSamplePixelSize
This geometries are not directly connected to anything. The TomoDetector
is responsible of updating them, with the up-to-date information from the sub systems.