pvlib compat with contrib.gen_coeffs.gen_iec_61853_from_sapm

pvlib 0.7.0 changed the sapm irradiance input from suns to W/m2 (https://pvlib-python.readthedocs.io/en/stable/whatsnew.html#v0-7-0-december-18-2019) and running `contrib/gen_coeffs/example.py` with pvlib >=0.7.0 fails as a result.  

Relevant line of code: https://github.com/SunPower/PVMismatch/blob/master/pvmismatch/contrib/gen_coeffs/__init__.py#L27