Fix error in the irradiance unit checker. (#844)

Author: dzimmanck

pvlib/pvsystem.py

@@ -1678,7 +1678,7 @@ def sapm(effective_irradiance, temp_cell, module):
     irrad_ref = 1000
     # TODO: remove this warning in v0.8 after deprecation period for change in
     # effective irradiance units, made in v0.7
-    if np.all(effective_irradiance) < 2.0:
+    if np.all(effective_irradiance < 2.0):
         import warnings
         warnings.warn('effective_irradiance inputs appear to be in suns.'
                       ' Units changed in v0.7 from suns to W/m2',