Get the angle of incidence on the system.

        Parameters
        ----------
        solar_zenith : float or Series.
            Solar zenith angle.
        solar_azimuth : float or Series.
            Solar azimuth angle.

        Returns
        -------
        aoi : Series
            The angle of incidence

def get_aoi(self, solar_zenith, solar_azimuth):
        """Get the angle of incidence on the system.

        Parameters
        ----------
        solar_zenith : float or Series.
            Solar zenith angle.
        solar_azimuth : float or Series.
            Solar azimuth angle.

        Returns
        -------
        aoi : Series
            The angle of incidence
        """

        aoi = irradiance.aoi(self.surface_tilt, self.surface_azimuth,
                             solar_zenith, solar_azimuth)
        return aoi