Added a very basic reflected light approximation to SecondaryEclipseLightCurve WIP

[christinahedges]

I saw there is a WIP to add SecondaryEclipseLightCurve to exoplanet, but that it doesn't include reflected/thermal phase curve light. I added a silly approximation to quickly add in a phase curve (just a cosine function), that has a lag keyword to shift the "hotspot" on the planet. This is a silly approximation because it doesn't work when there is a large lag, and it's not calculating the ingress/egress correctly.

I assume part of the WIP is to include starry reflected light. But just for now, because I wanted to use SecondaryEclipseLightCurve, I made this edit, and I thought I'd post it here in case anyone was interested in seeing. Not looking for this to be merged, but if anyone had comments about how to contribute to exoplanet better, I'd read them!

Here's an example use

import matplotlib.pyplot as plt
import exoplanet as xo
import numpy as np
t = np.arange(0, 2, 0.001)
period = 0.5

orbit = xo.orbits.KeplerianOrbit(period=period, r_star=1, m_star=1)
sec = xo.SecondaryEclipseLightCurve([0.2, 0.25], [0], 0.01).get_light_curve(orbit=orbit, r=0.1, t=t, reflected=True, lag=0.2)
plt.plot(sec.eval()[:, 0])