f and g functions for eccentric and true anomaly.

Hyperbolic and Parabolic not included

[skip ci]
f and g functions for hyperbolic anomaly

[skip ci]
f and g functions for parabolic anomaly
f & g functions test improvements
f and g time series

f and g series with four terms.

These are next to useless because of their low accuracy. More terms are
required to get any kind of decent accuracy.

References:
Herrick, Samuel: Astrodynamics, Vol. 1

[skip ci]
f & g time series

Taylor series expansion based on "Lagrange's fundamental invariants".

References:
Bate, Mueller, White: Fundamentals of Astrodynamics
Herrick: Astrodynamics

[skip ci]
f & g time series testing