Read the strenth and direction of a homogeneous extenal
magnetic field and of reduction factors (see p. 
) in
the form
HMG <hmag> <thmag> <phmag> [<kx> <ky> <kz>].
<hmag> is the magnetic field strength H in Tesla, the polar coordinates
<thmag> and <phmag> define the direction 
of the
magnetic field.