project_filter_model: Continue a run from a fitted filter model.
In rplzzz/CovMitigation: COVID-19 infection model for Virginia

Unlike fit_filter, this function just continues running the model forward in time, with no parameter updates. Final state and summary statistics are computed and appended to the ones computed for the original fit. History is appended if it is present in the original fit.

1	project_filter_model(filterfit, tfinal, dt = 1, tvintage1 = NA, dtvintage = 7)

`filterfit`	Structure of class filter-fit returned by `fit_filter`.
`tfinal`	Final time for the projection.
`dt`	Time step in days. Note that if `dt > 1`, then `tfinal` will not necessarily be in the final output.
`tvintage1`	Time for the first projection vintage. Having multiple vintages is only possible if history information was included in `filterfit`.
`dtvintage`	Time step between vintages.