A template instrument shows how to create a focal point monitor with an
instance of Monitor_nD and an EXTEND block on a preceeding Arm.
The instrument uses a point source and a thin lens to create a focus.
The working principle behind, is to bin the weight carried by each ray by the
z-coordinate (of a cartesian system defined by an Arm preceeding the Monitor_nD
instance) where the ray's trajectory passes closest to the Z-axis.
Note, the origin of the monitored z-coordinate is defined by the Arm on which
the EXTEND-block is attached (In this case fpt0). If the subsequent monitor
is placed elsewhere an implicit offset will be the result.
Parameters in boldface are required;
the others are optional.