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

include/matador/orm/session.hpp

@@ -165,6 +165,8 @@ public:
     return stmt_;
   }
 
+  template<typename BaseType>
+  static void on_base(const BaseType&) {}
   template<class Type>
   void on_primary_key(const char * /*id*/, Type &x, const utils::primary_key_attribute & /*attr*/) {
     stmt_.bind(binding_position_, x);