Describe the bug
Docstring for pvsystem.sapm states

    module : dict-like
        A dict, Series, or DataFrame defining the SAPM performance
        parameters. See the notes section for more details.

DataFrame input is not tested. The function errors when module parameter are supplied in a DataFrame.

To Reproduce
Steps to reproduce the behavior: see attached

Expected behavior
pvsystem.sapm should give correct results if parameters are in a DataFrame. Otherwise, we should remove the option from the docstring.

Versions:

• pvlib.__version__: 0.6.1
• pandas.__version__: 0.24.2
• python: 3.6.8

sapm_dataframe.txt