Front-side irradiance is not affected by horizon_band_angle

[kandersolar]

Originally posted in https://github.com/SunPower/pvfactors/issues/131:

Changing the horizon_band_angle input to HybridPerezOrdered doesn't seem to affect front-side irradiance nearly as much as it does rear-side. Code to reproduce:

from pvfactors.geometry import OrderedPVArray
from pvfactors.engine import PVEngine
from pvfactors.irradiance.models import HybridPerezOrdered
import pandas as pd
import numpy as np

times = pd.to_datetime(['2019-06-01 12:00'])
dni = np.array([600])
dhi = np.array([100])
solar_zenith = np.array([45])
solar_azimuth = np.array([270])
surface_tilt = np.array([45])
surface_azimuth = np.array([270])
albedo = np.array([0.25])

def build_report(pvarray):
    row = pvarray.ts_pvrows[1]
    return {
        'front': row.front.get_param_weighted('qabs')[0],
        'back': row.back.get_param_weighted('qabs')[0],
    }

for band_angle in [5, 15]:
    irradiance_model = HybridPerezOrdered(horizon_band_angle=band_angle)
    pvarray = OrderedPVArray(n_pvrows=3, pvrow_height=1, pvrow_width=1, axis_azimuth=180, gcr=0.5)
    engine = PVEngine(pvarray, irradiance_model=irradiance_model)
    engine.fit(times, dni, dhi, solar_zenith, solar_azimuth, surface_tilt, surface_azimuth, albedo)
    out = engine.run_full_mode(fn_build_report=build_report)
    print(f'band_angle = {band_angle}\t-->', out)

band_angle = 5  --> {'front': 737.1206177322375, 'back': 43.08209190810574}
band_angle = 15 --> {'front': 737.1517229315606, 'back': 50.38605014260793}

Looking at HybridPerezOrdered.transform, I guess the horizon band shading calculation for the front is done differently from the rear. In fact it's not clear to me that horizon band shading is calculated at all for the front side, but maybe I'm missing it. If it is, I would naively expect horizon band shading loss to be more or less the same for front and rear side.