Allow hyperbolic orbits to stay within the bounds of a plot

After #19 was fixed plotting only one hyperbolic orbit looks okay, but there can be cases when one wants it to do something like this:

This plot needed a patch in _generate_vals (hence tagging it as a bug), but it only works for cases where there’s a previous closed orbit plotted. The logic to handle both cases has to be more complex.

[astrojuanlu]

I stumbled upon this again while running the notebook I mentioned above. This is the code to reproduce with the current master:

ss_hyp = Orbit.from_vectors(
    Sun,
    [ -9.77441841e+07,  1.01000539e+08,  4.37584668e+07] * u.km,
    [ 23.75936985,-43.09599568, -8.7084724 ] * u.km / u.s,
)
ss_big = Orbit.from_vectors(
    Sun,
    [  4.62521605e+10,  1.55496163e+11, -4.18874038e+10] * u.km,
    [-0.46461615, 0.20385617, 0.24373322] * u.km / u.s
)

Plotting each of them separately works:

(notice the slight bug in the hyperbolic one?)

However, plotting them together shows only the big, elliptic orbit (regardless of the order):

frame = OrbitPlotter()

frame.plot(ss_big)
frame.plot(ss_hyp)

Therefore, it’s not enough to compute the limit nu_values for hyperbolic orbits inside the sample method: the plotting code should set them, depending on the bounds of the plot or some other parameter. Still not sure how to fix this.

[astrojuanlu]

Perhaps it’s not really something we can solve. Our current APIs allow you to clip the axis and change its limits, so perhaps that’s enough. I’m closing this.