reorder np.nan replacement, np.maximum in sapm_spectral_loss (#608)

Author: cwhanse

docs/sphinx/source/whatsnew/v0.6.1.rst

@@ -39,6 +39,8 @@ Enhancements
 * Created :py:func:`pvlib.iotools.read_surfrad` to read NOAA SURFRAD data. (:issue:`590`)
 * Created :py:func:`pvlib.iotools.read_midc` and :py:func:`pvlib.iotools.read_midc_raw_data_from_nrel`
   to read NREL MIDC data. (:issue:`601`)
+* Change :py:func:`pvlib.pvsystem.sapm_spectral_loss` to avoid numpy warning.
+
 
 Bug fixes
 ~~~~~~~~~

pvlib/pvsystem.py

@@ -1914,10 +1914,12 @@ def sapm_spectral_loss(airmass_absolute, module):
     am_coeff = [module['A4'], module['A3'], module['A2'], module['A1'],
                 module['A0']]
 
-    spectral_loss = np.maximum(0, np.polyval(am_coeff, airmass_absolute))
+    spectral_loss = np.polyval(am_coeff, airmass_absolute)
 
     spectral_loss = np.where(np.isnan(spectral_loss), 0, spectral_loss)
 
+    spectral_loss = np.maximum(0, spectral_loss)
+
     if isinstance(airmass_absolute, pd.Series):
         spectral_loss = pd.Series(spectral_loss, airmass_absolute.index)