Change output name for R^d vector

Write `R^d` instead of `vector[d]`. This is less confusing especially
w.r.t. error messages.

src/language/ASTNodeDataType.cpp

@@ -38,7 +38,7 @@ dataTypeName(const ASTNodeDataType& data_type)
     name = "R";
     break;
   case ASTNodeDataType::vector_t:
-    name = "vector[" + std::to_string(data_type.dimension()) + "]";
+    name = "R^" + std::to_string(data_type.dimension());
     break;
   case ASTNodeDataType::string_t:
     name = "string";