new leapfrog class
new leapfrog class
 update __all__
return times as well
update tests to use new integrator, allow ndim=1 arrays
add test for adaptive timestep
fix time parser for negative dtr
update sgr read-in to use new integrator
use new time_spec kwarg and fix energy plots
update to use new time_spec arg
new class for integrating particles and a satellite back together
fix tests, add underscore
pre-build array of positions and velocities, much faster
clean up data subdir
remove old test for core.py
new generic function for reading sim data
stupid whitespace
moved unitsystem class into data/
import unitsystem from data
non-working modifications for adaptive steps