McStas: single_crystal(powder=1) is not functional. McXtrace is OK

[farhi]

The single_crystal(powder=1) component does not provide the proper results. The equivalent McXtrace implementation works nicely.
I recommend comparing the two components, and fix the neutron side...

[farhi]

It seems to be related to line ~1348 in the McStas version:

        component_vx = vx;
        component_vy = vy;
        component_vz = vz;

which then overrides any powder mode rotation at line ~1543:

        vx = component_vx;
        vy = component_vy;
        vz = component_vz;

these lines are not present in the X-ray version.

[farhi]

The commit responsible for this change is:

• 9006146#diff-5098f992a4af7bef3fc30ace5e8da4a189b12f1dc68e0ff2e9854e6c82aaa02d
  and the corresponding issue is:
• Single crystal PG powder fix #1521

[farhi]

I recommend a revert...