Calculate the expectation-based Poisson statistic for each space-time window, given the initial data of counts and mus.
poisson_calculations(table, zones)A data.table with columns location, duration,
count, mu.
A list or set of zones, each zone itself a
set containing one or more locations of those found in table.
A data.table with columns zone, duration, statistic.
The column statistic contains the logarithm of the statistic for
each zone-duration combination.