ModelChain.pvwatts_losses creates self.losses instead of ModelChainResult.losses

[wholmgren]

@ncroft-b4 reports...

It looks like pvwatts_losses still creates a self.losses attribute.

Thanks for catching that. pvwatts_losses and no_extra_losses should assign values to ModelChainResults, and ModelChain._deprecated_attrs should include losses. We can address that in another issue/pr.

Originally posted by @wholmgren in #1168 (comment)