create interface between IncidenceMatrix and sts objects

[nickreich]

Note that the surveillance package uses the standard R sts object for multivariate modeling, e.g.,

library(surveillance)
data(fluBYBW)

seems like this should be easy enough to covert into an IncidenceMatrix, and would allow for easier interfacing to standard R time-series objects, especially as relates to the hhh4() spatial modeling functions.