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

include/matador/sql/generator.hpp

@@ -44,6 +44,8 @@ public:
     return columns;
   }
 
+  template<typename BaseType>
+  static void on_base(const BaseType&) {}
   template < class V >
   void on_primary_key(const char *id, V &, const utils::primary_key_attribute& /*attr*/) {
       if (has_many_to_many_) {