Implement parabolic orbits

[ajtribick]

Some notes from investigating how to do this...

Parameterize with $q$ instead of $a$ (probably this is why the EllipticalOrbit class stores pericenterDistance instead of semimajorAxis, which currently leads to extra division operations - probably makes sense to have ParabolicOrbit as a separate class and use semimajorAxis for elliptical/hyperbolic orbits. Add support for MeanMotion instead of Period (also useful for hyperbolic orbits)

From https://www.bogan.ca/orbits/kepler/orbteqtn.html

$$ M = D + \frac{D^3}{3} $$

$$ D = \tan \frac{\theta}{2} $$

$$ r = \frac{2q}{1 + \cos \theta} $$

Derived calculations:

Go from $M$ to $D$ by solving cubic:

$$ u = \sqrt[3]{ \frac{3M}{2} \pm \sqrt{\left(\frac{3M}{2}\right)^2 + 1} } $$

$$ D = u - \frac{1}{u} $$

Conversion to Cartesians:

$$ x = q\left( 1 - D^2 \right) $$

$$ y = 2qD $$

Velocities:

$$ \dot{x} = - \frac{2q\dot{M}D}{1 + D^2} $$

$$ \dot{y} = \frac{2q\dot{M}}{1 + D^2} $$

(check above derivations)