Polarising guide with two supermirrors sitting in a v-shape inside.
Author: Peter Christiansen
Date: July 2006
Models a rectangular guide with entrance centered on the Z axis and
with two supermirrors sitting in a v-shape inside.
The entrance lies in the X-Y plane. Draws a true depiction
of the guide with mirrors, and trajectories.
The polarisation is handled similar to in Monochromator_pol.
The reflec functions are handled similar to Pol_mirror.
The up direction is hardcoded to be along the y-axis (0, 1, 0)
Note that this component can also be used as a frame overlap-mirror
if the up and down reflectivities are set equal. In this case the wall
reflectivity (rPar) should probably be set to 0.
Reflectivity values can either come from datafiles or from
sets of parameters to the standard analytic reflectivity function commonly
used for neutron guides: