Added propagation code, removed twobody package
Changed the way quantities are handled in assertions, see
astropy/astropy#1930
Inverted logic in coe2rv
Units handling made more coherent
Now the handling of quantities with units is totally decoupled.
The `State` objects have fully knowledge of units, whereas the
functions of the twobody module don't. If all NumPy ufuncs worked
with units the functions would work equally with or without units;
with the current state of astropy.units they require raw values.