add post_constructor() method to computes for symmetry with fixes

src/compute.h

@@ -115,6 +115,7 @@ class Compute : protected Pointers {
   void modify_params(int, char **);
   virtual int modify_param(int, char **) { return 0; }
   virtual void reset_extra_dof();
+  virtual void post_constructor() {}
 
   void init_flags();
   virtual void init() = 0;

src/modify.cpp

@@ -1293,7 +1293,18 @@ Compute *Modify::add_compute(int narg, char **arg, int trysuffix)
     error->all(FLERR, utils::check_packages_for_style("compute", arg[2], lmp));
 
   compute_list = std::vector<Compute *>(compute, compute + ncompute + 1);
-  return compute[ncompute++];
+
+  // post_constructor() can call virtual methods in parent or child
+  //   which would otherwise not yet be visible in child class
+  // post_constructor() allows new compute to create other computes
+  // ncompute increment must come first so recursive call to add_compute within
+  //   post_constructor() will see updated ncompute
+
+  auto *newcompute = compute[ncompute];
+  ++ncompute;
+  newcompute->post_constructor();
+
+  return newcompute;
 }
 
 /* ----------------------------------------------------------------------