Tweak Makefile

Remove -lkepler -L. from LDLIBS and LDFLAGS.

Use $^ instead of $< to allow many object files in static libraries.
Implementation of Stumpff functions

Implementation of Stumpff functions c0..c3 using series, trigonometric
functions and fast quadruple angle formulae.

References:
Danby, J.M.A.: Fundamentals of Celestial Mechanics
Battin, R.: Introduction to the mathematics and methods of Astrodynamics
Bate, Mueller, White: Fundamentals of Astrodynamics