Implement hyperbolic anomaly conversion functions

[astrojuanlu]

🐞 Problem

We should have hyperbolic anomaly conversion functions, in the same way we have elliptic anomaly functions here:

https://github.com/poliastro/poliastro/blob/v0.7.0/src/poliastro/twobody/angles.py

Right now, using nu_to_M for eccentricity values higher than one gives NaN:

In [2]: from poliastro.twobody.angles import *

In [3]: from astropy import units as u

In [4]: nu_to_M(0.1 * u.rad, 1.5 * u.one)
/home/juanlu/.miniconda36/envs/py36/lib/python3.6/site-packages/astropy/units/quantity.py:641: RuntimeWarning: invalid value encountered in sqrt
  *arrays, **kwargs)
Out[4]: <Quantity nan rad>

🎯 Goal

This is important since Orbit.sample uses the angle conversion functions to derive appropriate time values for propagation, and right now it doesn't work for hyperbolic orbits.

💡 Possible solutions

We should implement these equations:

(from Curtis "Orbital Mechanics for Engineering Students")

See also: https://en.wikibooks.org/wiki/Astrodynamics/Time_of_Flight

However, there are limitations in the true anomaly values, so care should be taken:

(from Vallado "Fundamentals of Astrodynamics")

Finding references for validation proved to be a bit difficult, so I posted a question on Physics.SE here https://physics.stackexchange.com/q/365117/7641

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[astrojuanlu]

I'm labeling this as a bug to fix in 0.8.

[anhiga]

Closed by #276