include fittedvalues and resid properties in ThetaModelResults

[zach-clem]

Problem:
I'm trying to get the fitted values and the residuals for the theta model. This is possible for HoltWintersResults, UnobservedComponentsResults, SarimaxResults, and others. Since I'm trying all different approaches, I also need to get the fittedvalues and residuals for the ThetaModelResults

Solution:
ThetaModelResults would have a property of fittedvalues and resid, similar to the models mentioned above.