Basically, this makes it so that the resulting merged CylinderSurface
always has average phi == 0, i.e. symmetric phi sector around the local
x axis, without any extra phi shift.

Equivalent update of DiscSurface still pending

This is not actually needed, because we require the surfaces to not have
detector elements anyway, and in that case, the transform is local to
the surface anyway (and not alignable)

… in z mode