Incorrect units in major axis in docs

[PabloVD]

In the documentation about orbital elements, the major axis appears in km. However, it should be in meters, as is used in the examples such as in the BasicOrbit.

[schaubh]

SMA does not have to meters or kilometers. It just has to be consistent with the units of mu. The file orbitalMotion.py is now updated to clarify the units used.