Add orbit fitting

[dahlend]

Yes, I am adding orbit fitting, I got bored.

This is a very large change, there are basically 3 major additions with numerous small improvements.

• Add an Initial Orbit Determination method (IOD) - This will produce a series of plausible orbits for provided orbits. These plausible orbits are terrible, but are enough to allow the actual orbit determination to work.
• Fitting Orbits - This requires an initial guess (see IOD), and takes a set of observations.
• MCMC Orbit fitting - Meant for short arcs of observations of new objects, similar to SCOUT.

Along with these major additions comes a lot of new code that is required to implement them.

There is a new tutorial which shows how to do MCMC fitting, and 2 new examples, one for orbit fitting and the other for state transition matrix calculations.