Change hardcoded std::size_t into size_type so it can be customised.

Define VWR_SIZE_TYPE to the type you want to use as the template
index type. It defaults to std::size_t so if you don't do anything
you still get the old behaviour.

Also drop the std::index_range in vector_cast in favour of the custom
bt::number_range, which is already used elsewhere in the code.

test/unit/sample_vectors.hpp

@@ -62,7 +62,7 @@ namespace vwr {
 		typedef float scalar_type;
 		typedef SimpleVector2 higher_vector_type;
 
-		static scalar_type& get_at (std::size_t, vector_type& parVector) {
+		static scalar_type& get_at (size_type, vector_type& parVector) {
 			return parVector;
 		}
 	};