Fix total_forces_same_step flag for LAMMPS

So far is was erroneously set to true, but its effects were not fully
implemented in ABF.

lammps/src/COLVARS/colvarproxy_lammps.h

@@ -70,7 +70,8 @@ class colvarproxy_lammps : public colvarproxy {
  public:
 
   bool total_forces_enabled() const override { return total_force_requested; };
-  bool total_forces_same_step() const override { return true; };
+  // Total forces are saved at end of step, only processed at the next step
+  bool total_forces_same_step() const override { return false; };
   bool want_exit() const { return do_exit; };
 
   // perform colvars computation. returns biasing energy

src/colvarproxy_system.h

@@ -94,6 +94,7 @@ class colvarproxy_system {
   virtual bool total_forces_enabled() const;
 
   /// Are total forces from the current step available?
+  /// in which case they are really system forces
   virtual bool total_forces_same_step() const;
 
   /// Get the molecule ID when called in VMD; raise error otherwise