splited big file into smaller files by functionality

sort.go

@@ -0,0 +1,10 @@
+package interval
+
+import "sort"
+
+func (intvls *intervals) Sort() {
+	if !intvls.Sorted {
+		sort.Sort(ByLow(intvls.Intervals))
+	}
+	intvls.Sorted = true
+}