snvecR: Calculate Earth's Obliquity and Precession in the Past

Easily calculate precession and obliquity from an astronomical solution (defaults to ZB18a from Zeebe and Lourens (2019)) and assumed or reconstructed values for tidal dissipation (Td) and dynamical ellipticity (Ed). This is a translation and adaptation of the 'C'-code in the supplementary material to Zeebe and Lourens (2022), with further details on the methodology described in Zeebe (2022). The name of the 'C'-routine is 'snvec', which refers to the key units of computation: spin vector s and orbit normal vector n.