modernize fix vector

src/fix_vector.h

@@ -36,9 +36,17 @@ class FixVector : public Fix {
   double compute_array(int, int) override;
 
  private:
-  int nvalues;
-  int *which, *argindex, *value2index;
-  char **ids;
+  struct value_t {
+    int which;
+    int argindex;
+    std::string id;
+    union {
+      class Compute *c;
+      class Fix *f;
+      int v;
+    } val;
+  };
+  std::vector<value_t> values;
 
   bigint nextstep, initialstep;
 
@@ -47,8 +55,6 @@ class FixVector : public Fix {
   double *vector;
   double **array;
 };
-
 }    // namespace LAMMPS_NS
-
 #endif
 #endif