Pvlib-python: PVWatts_loss was applied on AC calculation

[yxh289]

Hi,
In the description of pvwatts_losses function, the losses are applied on DC side:
'Calculates DC power losses according the PVwatts model using pvwatts_losses() and self.losses_parameters.`'
However I found that in pvlib.modelchain.ModelChain.pvwatts_losses, PVwatts loss was applied to AC.
'def pvwatts_losses(self):
self.losses = (100 - self.system.pvwatts_losses()) / 100.
self.ac *= self.losses
return self'
When system have high DC/AC ratio, AC power output clips at a lower level as a result of these losses applied on the AC side.

[wholmgren]

thanks @yxh289. ~pull request welcome.~ In a rush to finish off 0.6.2 so going ahead with a PR.

[cwhanse]

Concur with the bug report. May have been due to coding following the older PVWatts description, rather than the v5 document. Or a simple typo.