Calculate magnetic field at a given location and time according to
the World Magnetic Model (WMM). Both the main field and secular variation
components are returned. This functionality is useful for physicists and
geophysicists who need orthogonal components from WMM. Currently, this package
supports annualized time inputs between 2000 and 2020. If desired, users can
specify which WMM version to use, e.g., the original WMM2015 release or the
recent out-of-cycle WMM2015 release. Methods used to implement WMM, including
the Gauss coefficients for each release, are described in the following
publications: Chulliat et al (2019)