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.

test/tools.jl

@@ -347,9 +347,14 @@ end
     @test 0 == length(suslis[1])
     @test 1 == length(suslis[2])
     @test 2 == length(suslis[3])
-    @test all(20 .== length(suslis[3:end-2]))
+    @test all(20 .== length.(suslis[4:end-5]))
     @test 8 == length(suslis[end-1])
     @test 6 == length(suslis[end])
 
+    flat_suslis = vcat(suslis...)
+    frame_indices = [s.header.frame_index for s in flat_suslis]
+    @test issorted(frame_indices)
+    @test frame_indices == sort([s.header.frame_index for s in f.online.summaryslices])
+
     close(f)
 end