cc: fix returning type of sel_types

Fix the following compiler warning:
```
/home/runner/work/deepmd-kit/deepmd-kit/source/api_c/src/c_api.cc:1336:17: warning: returning address of local temporary object [-Wreturn-stack-address]
  return (int*)&(dcm->dcm.sel_types())[0];
                ^~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
```

by returning the reference of sel_type.

It seems this function is not used anywhere, even in the test, so we don't have an chance to find out if there is possible segfault. (so this warning has no impact)

It seems DeepTensor has returned a reference since the beginning (#137). (perhaps because DeepTensor.sel_types is actually used) DeepTensor and DataChargeModifier have different returned types.

source/api_cc/include/DataModifier.h

@@ -84,7 +84,7 @@ class DipoleChargeModifierBase {
    * @brief Get the list of sel types.
    * @return The list of sel types.
    */
-  virtual std::vector<int> sel_types() const = 0;
+  virtual std::vector<int>& sel_types() const = 0;
 };
 
 /**
@@ -161,7 +161,7 @@ class DipoleChargeModifier {
    * @brief Get the list of sel types.
    * @return The list of sel types.
    */
-  std::vector<int> sel_types() const;
+  std::vector<int>& sel_types() const;
 
  private:
   bool inited;

source/api_cc/include/DataModifierTF.h

@@ -84,7 +84,7 @@ class DipoleChargeModifierTF : public DipoleChargeModifierBase {
    * @brief Get the list of sel types.
    * @return The list of sel types.
    */
-  std::vector<int> sel_types() const {
+  std::vector<int>& sel_types() const {
     assert(inited);
     return sel_type;
   };