Position and velocity in 3d

Transform position and velocity by multiplying with the orbit's
rotation matrix.

Add test to test acceleration and velocity.
Makefile instead of CMake
Add .travis.yml
Don't specify $(CC) in Makefile

Use the default value from environment variable.
Sanity tests for parabolic orbits
Initial position above periapsis in testing

Change orbit_from_state test to use initial values that are above
periapsis.
Test position, velocity at periapsis, apoapsis
Test case ordering based on Morton code
Build and testing improvements

numtest: add --list
numtest: change --last to be inclusive (<=)

Change eccentricity limits used in test cases.
Minor refactoring to improve branch coverage in kepler.c.
Velocity from true anomaly

Do not use the vis-viva equation (specific orbital energy) to calculate
magnitude of velocity.
Universal variable formulation
More universal variables

Add f & g expressions in terms of eccentric and true anomaly. And more
tests that fail.
Makefile for universal variable
Stumpff functions
Stumpff function derivative test