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

include/matador/object/join_columns_collector.hpp

@@ -24,6 +24,8 @@ public:
 
         return join_columns_;
     }
+    template<typename BaseType>
+    static void on_base(const BaseType&) {}
     template < class V >
     static void on_primary_key(const char * /*id*/, V &, const utils::primary_key_attribute& /*attr*/) {}
     static void on_revision(const char * /*id*/, uint64_t &/*rev*/) {}