splited big file into smaller files by functionality

add.go

@@ -0,0 +1,13 @@
+package interval
+
+func (intvls *intervals) Add(itvl *Interval) {
+	intvls.Intervals = append(intvls.Intervals, itvl)
+	intvls.reset()
+}
+
+func (intvls *intervals) reset() {
+	intvls.Sorted = false
+	intvls.GapsList = nil
+	intvls.OverlappedList = nil
+	intvls.MergeList = nil
+}