refactor: Define accessor to reset the colvar total force

Author: giacomofiorin

src/colvar.cpp

@@ -1409,7 +1409,7 @@ int colvar::calc()
     error_code |= calc_cvcs();
     if (!cvm::main()->proxy->total_forces_valid()) {
       // Zero out the colvar total force when atomic total forces are not available
-      ft.reset();
+      reset_total_force();
     }
     if (error_code != COLVARS_OK) return error_code;
     error_code |= collect_cvc_data();
@@ -1437,7 +1437,7 @@ int colvar::calc_cvcs(int first_cvc, size_t num_cvcs)
     error_code |= calc_cvc_total_force(first_cvc, num_cvcs);
   } else {
     // Zero out the colvar total force when atomic total forces are not available
-    ft.reset();
+    reset_total_force();
   }
   // atom coordinates are updated by the next line
   error_code |= calc_cvc_values(first_cvc, num_cvcs);
@@ -1687,6 +1687,7 @@ int colvar::calc_cvc_total_force(int first_cvc, size_t num_cvcs)
 int colvar::collect_cvc_total_forces()
 {
   if (is_enabled(f_cv_total_force_calc)) {
+
     ft.reset();
 
     for (size_t i = 0; i < cvcs.size();  i++) {

src/colvar.h

@@ -82,6 +82,12 @@ class colvar : public colvarparse, public colvardeps {
   /// subtracted.
   colvarvalue const & total_force() const;
 
+  /// Reset the total force to zero
+  inline void reset_total_force()
+  {
+    ft.reset();
+  }
+
   /// \brief Typical fluctuation amplitude for this collective
   /// variable (e.g. local width of a free energy basin)
   ///

src/colvar_gpu_calc.cpp

@@ -30,6 +30,9 @@ int colvarmodule_gpu_calc::cvc_calc_total_force(
   for (auto cvi = colvars.begin(); cvi != colvars.end(); cvi++) {
     // Calculate CVC total force
     if (!(*cvi)->is_enabled(colvardeps::f_cv_total_force_calc)) continue;
+    if (!total_force_valid) {
+      (*cvi)->reset_total_force();
+    }
     const bool do_total_force =
       use_current_step ?
        (*cvi)->is_enabled(colvardeps::f_cv_total_force_current_step) :
@@ -41,12 +44,8 @@ int colvarmodule_gpu_calc::cvc_calc_total_force(
       const auto all_cvcs = (*cvi)->get_cvcs();
       for (auto cvc = all_cvcs.begin(); cvc != all_cvcs.end(); ++cvc) {
         if (!(*cvc)->is_enabled(colvardeps::f_cvc_active)) continue;
-        if ((*cvc)->is_enabled(colvardeps::f_cvc_active)) {
-          if (total_force_valid) {
-            (*cvc)->calc_force_invgrads();
-          } else {
-            // TODO: (*cvc)->ft is a protected member. How could I do the reset?
-          }
+        if ((*cvc)->is_enabled(colvardeps::f_cvc_active) && total_force_valid) {
+          (*cvc)->calc_force_invgrads();
         }
       }
       if (colvar_module->debug()) {