provide detector positioning example; repair AxesInDifferentUnits

A RectangularDetector can be placed normal to the z axis, to the transmitted beam, or to the reflected beam. All this is already covered by examples and tests, but these can be improved. They should show a rich scattering pattern so that one validate at once glance that differently positioned detectors show different regions of the pattern.

In current main branch, example scatter2d/AxesInDifferentUnits is broken because it involves removed c'tor FlatDetector.