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.

.gitlab-ci.yml

@@ -26,6 +26,7 @@ before_script:
         Pkg.Registry.add(RegistrySpec(url = "https://git.km3net.de/common/julia-registry"))'
       julia --project=@. -e '
         using Pkg
+        Pkg.resolve()
         Pkg.build()
         Pkg.test(coverage=true)'