Tweak Makefile

Remove -lkepler -L. from LDLIBS and LDFLAGS.

Use $^ instead of $< to allow many object files in static libraries.
Lagrangian coefficients (f and g functions)

Implement Lagrangian coefficients (f and g functions) using time series,
true anomaly, eccentric anomaly, hyperbolic anomaly and parabolic
anomaly.

References:
Bate, Mueller, White: Fundamentals of Astrodynamics
Herrick: Astrodynamics, Vol. 1