Set known flexion angle from digitisation

Assume digitisation angle is 0. Add option to `Digitisation` to set a digitisation angle. This is then used to define the true zero.

Leave the option where we set flexion min and max manually.

Start by locating the Digitisation entry point and the existing option for setting flexion minimum and maximum values. Add the digitisation-angle option so it defines the true zero while preserving the manual flexion min/max option; done means both configuration paths remain available.

Written by the indexing model from the issue text.