add get_type_map for c++ api

Author: njzjz

source/lib/include/NNPInter.h

@@ -59,6 +59,7 @@ class NNPInter
   int numb_types () const {assert(inited); return ntypes;};
   int dim_fparam () const {assert(inited); return dfparam;};
   int dim_aparam () const {assert(inited); return daparam;};
+  void get_type_map (std::string & type_map);
 private:
   Session* session;
   int num_intra_nthreads, num_inter_nthreads;

source/lib/src/NNPInter.cc

@@ -677,6 +677,11 @@ compute (ENERGYTYPE &			dener,
     run_model (dener, dforce_, dvirial, datom_energy_, datom_virial_, session, input_tensors, nnpmap, nghost);
 }
 
+void
+NNPInter::
+get_type_map(std::string & type_map){
+    type_map = get_scalar<std::string>("model_attr/tmap");
+}