Experimental branch using angle as a dimension.

Added dim_angle, Angle concept etc.
Not really for merging, but fun to play with.(The main problem is that it doesnt conform to SI)
See https://github.com/mpusz/units/issues/99
Could try adding degrees etc in same way as non-si units

example/experimental_angle.cpp

@@ -0,0 +1,22 @@
+
+
+
+#include <units/physical/si/length.h>
+#include <units/physical/si/torque.h>
+#include <units/physical/si/energy.h>
+#include <units/format.h>
+#include <iostream>
+
+using namespace units;
+using namespace units::physical::si::literals;
+
+int main()
+{
+   auto torque = 20.0q_Nm;
+   auto energy = 20.0q_J;
+
+   physical::Angle angle = torque/energy;
+
+   std::cout << angle <<'\n';
+}
+