First cut of Body class.
First cut of n-body system.
Frame: does it even make sense?
Whatever.
Merge branch 'master' into Physics

Conflicts:
	integrators/symplectic_integrator.hpp
	integrators/symplectic_partitioned_runge_kutta_integrator.hpp
	integrators/symplectic_partitioned_runge_kutta_integrator_body.hpp
Using std::bind.
More implementation, a short test.
Test compiling.
Initialize integrator parameters.
Solution extraction.
Test failing.
Mathematica printing.
Earth-Moon
Mass, speed.
Trying to understand the 2-body problem.
It works...
Readying for review.
Failing test.
Formatting.
Nicer API.