Remove times kwarg from ModelChain.prepare_inputs and ModelChain.run_model

[wholmgren]

The ModelChain.run_model and ModelChain.prepare_inputs methods times kwarg is nearly pointless. I'm guessing/hoping that the vast majority of ModelChain users supply a weather DataFrame with a meaningful DatetimeIndex. So, it would greatly simplify the API if we removed the times kwarg and inspected weather.index whenever we need timestamp info (e.g. solar position). ModelChain is committed to time series modeling using pandas, so I think it's reasonable to require users to supply a meaningful index.

Current signatures:

    def prepare_inputs(self, times=None, weather=None):
    def run_model(self, times=None, weather=None):

proposed signatures:

    def prepare_inputs(self, weather):
    def run_model(self, weather):