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

include/matador/sql/internal/pk_reader.hpp

@@ -14,6 +14,8 @@ public:
     access::process(*this, obj);
   }
 
+  template<typename BaseType>
+  static void on_base(const BaseType&) {}
   template<typename ValueType>
   void on_primary_key(const char *id, ValueType &value, const utils::primary_key_attribute& attr);
   void on_revision(const char * /*id*/, uint64_t &/*rev*/) {