Add Summaryslice interval iterator helper

This adds a new type called SummarysliceIntervalIterator which allows to iterator over chunks of timeslices of a given time interval. This is useful if someone wants to analyse e.g. 30 second intervals. The summaryslices are returned as Vector{Summaryslice} and are time-sorted as well.

docs/src/api.md

@@ -131,6 +131,7 @@ CHClient
 
 ### General tools
 ```@docs
+SummarysliceIntervalIterator
 getevent
 categorize
 nthbitset