Added radius to bodies, improved Maneuver signature
Now building Maneuver objects is less verbose and has units
checking. Also changed README build status icon.
Individual State elements, circular helper
Moved maneuver computation, ensure dvs are vectors
Now the maneuvers are computed through the Maneuver class, making
it consistent with the recent changes made to State. Besides,
the delta-V must be vectors so a failing test was added.