add extract to fix_group

this will allow users to define groups that are by default created internally by fixes, etc.

src/fix_group.cpp

@@ -251,3 +251,13 @@ void FixGroup::set_group()
 
   if (varflag) memory->destroy(var);
 }
+
+/* ---------------------------------------------------------------------- */
+
+void *FixGroup::extract(const char *str, int &unused)
+{
+  if (strcmp(str,"property") == 0 && propflag) return (void *) idprop;
+  if (strcmp(str,"variable") == 0 && varflag) return (void *) idvar;
+  if (strcmp(str,"region") == 0 && regionflag) return (void *) idregion;
+  return NULL;
+}

src/fix_group.h

@@ -33,6 +33,7 @@ class FixGroup : public Fix {
   void setup(int);
   void post_integrate();
   void post_integrate_respa(int,int);
+  void *extract(const char *,int &);
 
  private:
   int gbit,gbitinverse;
@@ -69,7 +70,7 @@ Self-explanatory.
 
 E: Per atom property for group dynamic does not exist
 
-UNDOCUMENTED
+Self-explanatory.
 
 E: Group dynamic parent group cannot be dynamic
 
@@ -79,10 +80,6 @@ E: Variable for group dynamic is invalid style
 
 The variable must be an atom-style variable.
 
-E: Per-atom property for group dynamic does not exist
-
-UNDOCUMENTED
-
 W: One or more dynamic groups may not be updated at correct point in timestep
 
 If there are other fixes that act immediately after the initial stage