Add Inner Ring code

pkg/core/netmap/epoch/math.go

@@ -0,0 +1,12 @@
+package epoch
+
+// Sum returns the result of
+// summing up two Epoch.
+//
+// Function defines a binary
+// operation of summing two Epoch.
+// Try to avoid using operator
+// "+" for better portability.
+func Sum(a, b Epoch) Epoch {
+	return FromUint64(ToUint64(a) + ToUint64(b))
+}