forecast 9.0.0

• ets() now allows missing values in the time series (#952)
• Added mean_model() and forecast.mean_model()
• Added rw_model() and forecast.rw_model() (m-muecke, #969)
• Added spline_model() and forecast.spline_model() (#1013)
• Added theta_model() and forecast.theta_model() (#1014)
• Added croston_model() and forecast.croston_model() (#1015)
• Added simulated and bootstrapped prediction intervals to more models (#1040)
• Added parallelization for nnetar (m-muecke, #346)
• More consistent handling of biasadj across models
• accuracy() rewritten to use S3 methods for models and remove accuracy.default() (#912)
• Bug fixes and performance improvements
• Documentation improvements