Find solar noon directly from maximum clearsky POA

When identifying the tilt and azimuth in `system.orientation()` we
calculate solar noon on each day by fitting a quadratic to the power
or irradiance measurements and comparing this to solar noon for
clearsky POA irradiance at different orientations. Rather than fitting
a quadratic to the daily clearsky POA irradiance and using the vertex
of the quadratic as solar noon this commit uses the maximum POA value
for the day. The idea is that curve fitting is unnecessary---because
clearsky POA has no noise or interference we can identify solar noon
directly from the data. This matches the methodology in PVFleets QA
and should improve performance; however, using the maximum POA
directly can reduce the accuracy of the tilt & azimuth that are
returned if the frequency of clearsky data is low. In general the best
accuracy can be obtained by passing one-minute clearsky and solar
position data.

Because of the reduced accuracy with low-frequency clearsky data the
unit tests needed to be updated. Rather than extending the range of
'okay' tilt/azimuth values I elected to pass one minute clearsky. For
one-minute data we know that the accuracy should be +/- 10 degrees.

pvanalytics/system.py

@@ -70,7 +70,7 @@ def orientation(power_or_poa, daytime, tilts, azimuths,
                 dni=dni
             ).poa_global
             poa_azimuths = azimuth_by_minute[
-                _peak_times(poa[solar_zenith < 70])
+                _group.by_day(poa).idxmax()
             ]
             sum_of_squares = sum(
                 (poa_azimuths.values - modeled_azimuth.values)**2

pvanalytics/tests/test_system.py

@@ -1,4 +1,5 @@
 """Tests for system paramter identification functions."""
+import pytest
 import pandas as pd
 from pvlib import irradiance
 from pvanalytics import system
@@ -9,7 +10,34 @@ def _assert_within(x, expected, margin):
     assert x in range(expected - margin, expected + margin + 1)
 
 
-def test_simple_poa_orientation(clearsky_year, solarposition_year):
+@pytest.fixture(scope='module')
+def fine_index(clearsky_year):
+    """DatetimeIndex for the same period as `clearsky_year` but with 1
+    minute frequency."""
+    return pd.date_range(
+        start=clearsky_year.index.min(),
+        end=clearsky_year.index.max(),
+        freq='T'
+    )
+
+
+@pytest.fixture(scope='module')
+def fine_clearsky(albuquerque, fine_index):
+    """Clearsky in Albuquerque with 1 minute timestamp spacing."""
+    return albuquerque.get_clearsky(
+        fine_index,
+        model='simplified_solis'
+    )
+
+
+@pytest.fixture(scope='module')
+def fine_solarposition(albuquerque, fine_index):
+    """Solar position in Albuquerque with 1 minute timestamp spacing."""
+    return albuquerque.get_solarposition(fine_index)
+
+
+def test_simple_poa_orientation(clearsky_year, solarposition_year,
+                                fine_clearsky, fine_solarposition):
     poa = irradiance.get_total_irradiance(
         surface_tilt=15,
         surface_azimuth=180,
@@ -21,10 +49,10 @@ def test_simple_poa_orientation(clearsky_year, solarposition_year):
         poa['poa_global'],
         tilts=range(5, 25, 5),
         azimuths=range(150, 200, 5),
-        solar_zenith=solarposition_year['apparent_zenith'],
-        solar_azimuth=solarposition_year['azimuth'],
+        solar_zenith=fine_solarposition['apparent_zenith'],
+        solar_azimuth=fine_solarposition['azimuth'],
         daytime=solarposition_year['apparent_zenith'] < 87,
-        **clearsky_year
+        **fine_clearsky
     )
     _assert_within(azimuth, 180, 10)
     _assert_within(tilt, 15, 10)
@@ -45,7 +73,7 @@ def test_ghi_tilt_zero(clearsky_year, solarposition_year):
 
 
 def test_azimuth_different_index(clearsky_year, solarposition_year,
-                                 albuquerque):
+                                 fine_clearsky, fine_solarposition):
     """Can use solar position and clearsky with finer time-resolution to
     get an accurate estimate of tilt and azimuth."""
     poa = irradiance.get_total_irradiance(
@@ -55,16 +83,6 @@ def test_azimuth_different_index(clearsky_year, solarposition_year,
         solar_zenith=solarposition_year['apparent_zenith'],
         solar_azimuth=solarposition_year['azimuth']
     )
-    fine_index = pd.date_range(
-        start=clearsky_year.index.min(),
-        end=clearsky_year.index.max(),
-        freq='1T'
-    )
-    fine_solarposition = albuquerque.get_solarposition(fine_index)
-    fine_clearsky = albuquerque.get_clearsky(
-        fine_index,
-        model='simplified_solis'
-    )
     azimuth, tilt = system.orientation(
         poa['poa_global'],
         daytime=solarposition_year['apparent_zenith'] < 87,