Create geo-aggregation extension of tsibble and demonstrate its use in aggregation vignette

The basic idea here is to do for geography what `tsibble` does for time. I will elaborate on this below. In particular, a key outcome of this issue would be to demonstrate geographic aggregation functionality in the second half of the `aggregate.Rmd` vignette. But unlike in https://github.com/cmu-delphi/epitools/issues/24, where the only task was to write the demo, in this case one would first need to actually develop the geographic aggregation functionality itself. I sketch out here a particular approach to this, which would involve writing a new separate R package that would inherit from `tsibble`, as described below.