Add the ability to construct arbitrary orbit objects

[pand5461]

I've noticed that orbit objects are not attached to existing orbits or maneuver nodes. One can create a maneuver node, save nextnode:orbit into a variable and delete the node. The variable still holds the orbit, although it's nowhere in the flight plan.

So, it could be useful to create orbital patches with user-defined parameters without maneuver node trickery. One example is the PEG algorithm where an auxiliary orbit is used to estimate gravity losses during a burn.

[ThunderousEcho]

Working on this

[ThunderousEcho]

Done on my branch.