introduced field_attributes shortcuts and added method on_base() to all processor classes

test/models/category.hpp

@@ -2,6 +2,7 @@
 #define QUERY_CATEGORY_HPP
 
 #include "matador/utils/access.hpp"
+#include "matador/utils/field_attributes.hpp"
 
 #include <string>
 
@@ -15,7 +16,7 @@ struct category {
     namespace field = matador::access;
     using namespace matador::utils;
     field::primary_key(op, "id", id);
-    field::attribute(op, "name", name, 255);
+    field::attribute(op, "name", name, UniqueVarChar255);
   }
 };
 }