extend_filter_model: Extend a filter model with new data.
In rplzzz/CovMitigation: COVID-19 infection model for Virginia

This function uses the information in a filter model structure to set up the input for fit_filter. The observed data is supplemented with a new dataset, and the fit is continued from the final state stored in the model.

1	extend_filter_model(filterfit, newobs, tfin = NULL)

`filterfit`	Structure of class filter-fit returned by `fit-filter`.
`newobs`	Observed dataset to use to extend the fit. This dataset should have some observations at later times than the final time in the input model. Data from times earlier than the final time in the input model will be ignored.
`tfin`	Last time to use to fit the new model. Default is to use the time of the last observation.