Rectangular box with constant B field along y-axis (up). The
component can be rotated to make either a guide field or a spin
flipper. A neutron hitting outside the box opening or the box sides
Example: Pol_constBfield(xwidth=0.1, yheight=0.1, zdepth=0.2, fliplambda=5.0)
Parameters in boldface are required;
the others are optional.