ModelChain.prepare_inputs can succeed with missing dhi

[wfvining]

From the docstring for ModelChain.prepare_inputs() I believe the method should fail if weather does not have a dhi column.

The validation checks for 'ghi' twice, but not 'dhi'

pvlib-python/pvlib/modelchain.py

Line 1136 in 11c356f

self._verify_df(weather, required=['ghi', 'dni', 'ghi'])