v1.8 Photon Epoch

Having read a bunch of papers on Lambert's problem (one of which was this), I decided to implement a Lambert's problem solver as a utility. It it not fail-proofed 100%, but it does work for most cases.

I also read some paper about the general relativitstic effects acting on the orbits of Galileo satellites. So, as expressed in the vectorial form in the paper, I implemented some general relativistic effects. These are tiny effects in the grand scheme of things, but I suggest great care as they are implemented in an extremely simplistic way and may not be the best representation of all general relativistic cases. Regardless, my results and the paper's results seemed to agree when I did test my implementation.

Finally, I compiled results for another orbit propagator validation case to extend the validation documentation. This time, it was the Geocentric orbit simulation of the International Space Station.

Full Changelog: v1.7...v1.8