add l1 integral symbolic calculation script

scripts/symbolic_calculations.jl

+using Reduce
+
+# explicitly calculate l1 integral over reference triangle with first order
+# lagrange discretization
+function l1triangle()
+    integrand_2 = :(abs(u0 * (1-x1-x2) + u1 * x1 + u2 * x2))
+
+    integral_2 = Reduce.Algebra.int(integrand_2, :x2)
+    integrand_1 = Reduce.Algebra.:-(
+        Reduce.Algebra.sub(:x2 => :(1 - x1), integral_2),
+        Reduce.Algebra.sub(:x2 => 0, integral_2))
+
+    integral_1 = Reduce.Algebra.int(integrand_1, :x1)
+    result = Reduce.Algebra.:-(
+        Reduce.Algebra.sub(:x1 => 1, integral_1),
+        Reduce.Algebra.sub(:x1 => 0, integral_1))
+
+    return result
+end