Bugfix for Colvars library (update to version 2019-08-05)

Bugfix for group2CenterOnly (coordNum option):
https://github.com/Colvars/colvars/pull/278

lib/colvars/colvaratoms.h

@@ -207,6 +207,12 @@ public:
   /// \brief Remove an atom object from this group
   int remove_atom(cvm::atom_iter ai);
 
+  /// Set this group as a dummy group (no actual atoms)
+  int set_dummy();
+
+  /// If this group is dummy, set the corresponding position
+  int set_dummy_pos(cvm::atom_pos const &pos);
+
   /// \brief Print the updated the total mass and charge of a group.
   /// This is needed in case the hosting MD code has an option to
   /// change atom masses after their initialization.