Not clear enough that distances need to be set on Y axis in 03_where_is_my_reader

[lauramurgatroyd]

The CIL convention is that distances are set along the Y axis.

In this notebook, for exercise C I tried:

geometry = AcquisitionGeometry.create_Cone2D(source_position=[0,0], detector_position=[source_to_detector_distance, 0], rotation_axis_position=[source_to_object_distance, 0])

This gives an error:

ZeroDivisionError: float division by zero

I don't think this is at all clear to the user what they have done wrong.

The solution is:

geometry = AcquisitionGeometry.create_Cone2D(source_position=[0,0],
                                             detector_position=[0,source_to_detector_distance],
                                             rotation_axis_position=[0,source_to_object_distance])

[lauramurgatroyd]

Previously, we did add a 'common mistakes' section, but this appears at the end of exercise D, so wouldn't be seen until later. Perhaps we can move it to be earlier on?