Cylindrical angles are not calculated correctly for grouped detectors

[mantid-builder]

Original Reporter: Russell Taylor

If you group two detectors (or banks of detectors) that are symmetrically on opposite sides of the beamline a call to detectorTwoTheta gives 0, which is not what we want!
This is because they are averaged in cartesian coordinates before conversion to polars.

Also, the view detectors option in qtiplot show the absolute r,theta,phi values when it would be better to show them relative to the sample position (in most cases the sample is at the origin, but not always).

[mantid-builder]

This issue was originally trac ticket 398