fieldAtTime :: (Fractional a, Eq a) => MagneticModel a -- ^ Magnetic field model
            -> a -- ^ Time since model epoch (year)
            -> SphericalHarmonicModel a -- ^ Spherical harmonic model of magnetic field at specified time. Field in nT, reference radius in km
fieldAtTime m t = (fieldAtEpoch m) ^+^ (t *^ secularVariation m)
fieldAtTime :: (Fractional a, Eq a) => MagneticModel a -- ^ Magnetic field model
            -> a -- ^ Time since model epoch (year)
            -> SphericalHarmonicModel a
fieldAtTime m t = (fieldAtEpoch m) ^+^ (t *^ secularVariation m)
fieldAtTime m t = (fieldAtEpoch m) ^+^ (t *^ secularVariation m)