A test instrument to check that supplying a magnetic field as a vector field point cloud
is working. The field referred to by the default input parameter MF is 10 \mu T along the z-axis
at entry and flips to l0 \mu T along the negative z-axis halfway through
N.b. this example will not be run by autotest since it reuireq the presence of the file
flipfield.dat in the working directory.
Parameters in boldface are required;
the others are optional.