HRRR_ESRL output_variable list is wrong #557
Description
Describe the bug
See https://lgtm.com/projects/g/pvlib/pvlib-python/snapshot/dist-1794124795-1535630266160/files/pvlib/forecast.py?sort=name&dir=ASC&mode=heatmap&showExcluded=false#xcbb7bc48300cd9bf:1
The list is missing a comma, so the strings are concatenated. This results in a KeyError in the ForecastModel.process_data method.
pvlib.__version__: 0.5.2