Support axes labelled qy, qz; restore example AxesInDifferentUnits

Axes labelled qy, qz are required for off-specular, and at least nice-to-have for SAS.

Minimally, provide conversion

• from phi,alpha to qy, qz for SAS,
• from alpha_i,alpha_f to qy, qz for offspec.

Possibly also support alternative constructor in GUI

• for Detector2D and q-based controls for GUI,

• for OffSpec.

• Resurrect and adapt example AxesInDifferentUnits (ref/result/ex/axes-in-different-units)

Typical experimental result:

Image courtesy of Phase III Physics LLC. Multilayer sample with some nonspecular issues. Data taken on the BL-04A magnetism reflectometer at ORNL. Features (left and right lobes) seem to be from surface roughness, or strain. Roughness depends on sample thickness, for a possible model see Pleshanov Nucl Instr Meth A 524, 273 (2004), eq 7.