Add orbital elements derived parameters
Remove redundant error checking code
Add test of orbital element code
Update orbit to use meters not km; perigee_radius insted of orbit_perigee
Update pseudo-msids docs
Update index.rst to mention derived params by class
Fix RST docstring formatting error (cannot use sections)
Update version to 0.31
Remove leftover debug print statement
Merge pull request #78 from sot/orbit-elements

Add orbital elements as derived parameters