1. Updated Gauss coefficients for WMM2020.

2. Significantly improved numerical stability by using closed-form equation for associated Legendre polynomials, instead of recursion.
3. Improved speed by ~7x by using multidimensional arrays.
4. Per WMM2020, this version now displays a blackout zone warning when the horizontal intensity is below 6000 nT.
5. In addition to orthogonal magnetic field components, GetMagneticFieldWMM returns the magnetic field elements h, f, i, & d as well as their secular variation.

CRAN release v1.0.0