Add mathutil package and constants

Introduce a new package for math utilities, including an Add function
and Euler's number. Also, add constants for Celsius conversion and basic
arithmetic.

mathutil/mathutil.go

@@ -0,0 +1,7 @@
+package mathutil
+
+var EulersNumber float64 = 2.7182818284590452353602874713526624977572470936999595749669676277240766303535475945713821785251664274
+
+func Add(a, b int) int {
+	return a + b
+}