Takes a reflectivity (default=1) as input and reflects rays in a ideal geometry
The mirror is positioned such that the a-axis of the mirror ellipsoid is on the
z-axis, the b-axis is along the y-axis and the c is along the x-axis.
The reference point of the mirror is the ellipsoid centre, offset by one half-axis along the y-axis.
(See the component manual for a drawing). Furthermore, the mirror centre may be shifted by z_offset
along the z-axis ( -c < z_offset < c ).
This means that to position the mirror correctly.
the user positions the ellipsoid governing the mirror shape, not the mirror itself.
Parameters in boldface are required;
the others are optional.