Integrate equinoctial elements in Orbit class

[togry]

When adding Equinoctial elements in addition to Keplerian and Cartesian elements, keeping track of which ones are valid and which need updating is more involved, and may require a bit of thought before diving into implementation.

Also, since the Keplerian elements are singular whenever i=0 or e=0, the approach in the present code (always converting to/from Keplerian elements) defeats the purpose of using non-singular elements in the first place. This must be revisited also, if non-singular elements are to add value.

[danielk333]

partially fixed in #3b9b150 by implementing Equinoctial elements transforms