cumulative poisson evaluations

[wsavran]

we need to support the cumulative consistency tests from csep1. the function should accept a list of forecasts and return a cumulative test result.

it might be possible to create a single function that could support every poisson test. effectively, the cumulative forecast introduces the time dimension. some care must be taken to handle missing time windows.

right now, this can be implemented by users, but should be included as a standard function along with associated testing.