CONAN.utils.ellipsoidal_variation_signal
========================================

.. py:function:: CONAN.utils.ellipsoidal_variation_signal(phi, A_ev=0, f1_ev=0, inc_rad=1.571)

   Calculate the ellipsoidal variation signal of a planet as a function of phase angle.
   The equation is given as F = Aev * (1 - cos(2*phi) - f1*cos(phi) - f2*cos(3*phi)).
   The semi-amplitude of the ellipsoidal variation is A, f1 and f2 are fractional coefficients controlling
   the fundamental and second harmonics respectively. see eq.8 of Esteves+2013 (https://iopscience.iop.org/article/10.1088/0004-637X/772/1/51)

   :param phi: phase angle (2*pi*phase for circular orbit) or true anomaly+omega-pi/2 in radians.
   :type phi: array-like
   :param A_ev: semi-amplitude of the ellipsoidal variation signal
   :type A_ev: float
   :param f1_ev: fractional coefficient controlling the fundamental harmonic
   :type f1_ev: float
   :param inc_rad: inclination of the planet orbit in radians. Default is 1.571 (90 degrees)
   :type inc_rad: float

   :returns: **F_ev** -- ellipsoidal variation signal as a function of phase
   :rtype: array-like