GDA: Axis constraint Scannables naming convention is unintuitive ('You' engine)

Where an axis (e.g. gamma) may be fixed by a constraint an _con Scannable is created rather than an Scannable in order to avoid a namespace clash with that axis' actual Scannable. This is unintuitive.

One solution is for Diffcalc to read up the last set value for an axis when that axes is constrained.

However if the axis is Scanned while hkl is held still this would result in two motor moves. This could be resolved by making the physical axis Scannables the same level as the hkl Scannable and treating them as a group, although this is a pretty heavy solution.