core: add missing explicit

src/multibody/pool/model.hpp

@@ -36,7 +36,7 @@ namespace pinocchio
     /// \param[in] model input model used for parallel computations.
     /// \param[in] pool_size total size of the pool.
     ///
-    ModelPoolTpl(const Model & model,
+    explicit ModelPoolTpl(const Model & model,
                           const int pool_size = omp_get_max_threads())
     : m_model(model)
     , m_datas((size_t)pool_size, Data(model))