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

include/matador/object/object_generator.hpp

@@ -74,6 +74,8 @@ public:
         return obj;
     }
 
+    template<typename BaseType>
+    static void on_base(const BaseType&) {}
     template < class Type >
     void on_primary_key(const char *, Type &x, const utils::primary_key_attribute& attr);
     void on_revision(const char *id, uint64_t &rev);