refactor: use __restrict instead of __restrict__

Wikipedia says "__restrict" is recognized by all three compilers. Let me
try it.

src/colvar_rotation_derivative.h

@@ -1,10 +1,6 @@
 #ifndef COLVAR_ROTATION_DERIVATIVE
 #define COLVAR_ROTATION_DERIVATIVE
 
-#ifdef _MSC_VER
-#define __restrict__
-#endif
-
 #include "colvartypes.h"
 #include <type_traits>
 #include <cstring>
@@ -13,7 +9,7 @@
 template <typename T, typename std::enable_if<std::is_same<T, cvm::atom_pos>::value, bool>::type = true>
 inline void read_atom_coord(
   size_t ia, const std::vector<T>& pos,
-  cvm::real* __restrict__ x, cvm::real* __restrict__ y, cvm::real* __restrict__ z) {
+  cvm::real* __restrict x, cvm::real* __restrict y, cvm::real* __restrict z) {
   *x = pos[ia].x;
   *y = pos[ia].y;
   *z = pos[ia].z;
@@ -22,7 +18,7 @@ inline void read_atom_coord(
 template <typename T, typename std::enable_if<std::is_same<T, cvm::atom>::value, bool>::type = true>
 inline void read_atom_coord(
   size_t ia, const std::vector<T>& pos,
-  cvm::real* __restrict__ x, cvm::real* __restrict__ y, cvm::real* __restrict__ z) {
+  cvm::real* __restrict x, cvm::real* __restrict y, cvm::real* __restrict z) {
   *x = pos[ia].pos.x;
   *y = pos[ia].pos.y;
   *z = pos[ia].pos.z;
@@ -334,9 +330,9 @@ struct rotation_derivative {
   template <bool use_dl, bool use_dq, bool use_ds>
   void calc_derivative_impl(
     const cvm::rvector (&ds)[4][4],
-    cvm::rvector* __restrict__ const dl0_out,
-    cvm::vector1d<cvm::rvector>* __restrict__ const dq0_out,
-    cvm::matrix2d<cvm::rvector>* __restrict__ const ds_out) const {
+    cvm::rvector* __restrict const dl0_out,
+    cvm::vector1d<cvm::rvector>* __restrict const dq0_out,
+    cvm::matrix2d<cvm::rvector>* __restrict const ds_out) const {
     if (use_ds) {
       // this code path is for debug_gradients, so not necessary to unroll the loop
       *ds_out = cvm::matrix2d<cvm::rvector>(4, 4);
@@ -469,9 +465,9 @@ struct rotation_derivative {
     */
   template <bool use_dl, bool use_dq, bool use_ds>
   void calc_derivative_wrt_group1(
-    size_t ia, cvm::rvector* __restrict__ const dl0_1_out = nullptr,
-    cvm::vector1d<cvm::rvector>* __restrict__ const dq0_1_out = nullptr,
-    cvm::matrix2d<cvm::rvector>* __restrict__ const ds_1_out = nullptr) const {
+    size_t ia, cvm::rvector* __restrict const dl0_1_out = nullptr,
+    cvm::vector1d<cvm::rvector>* __restrict const dq0_1_out = nullptr,
+    cvm::matrix2d<cvm::rvector>* __restrict const ds_1_out = nullptr) const {
       // if (dl0_1_out == nullptr && dq0_1_out == nullptr) return;
       cvm::real a2x, a2y, a2z;
       // we can get rid of the helper function read_atom_coord if C++17 (constexpr) is available
@@ -495,9 +491,9 @@ struct rotation_derivative {
     */
   template <bool use_dl, bool use_dq, bool use_ds>
   void calc_derivative_wrt_group2(
-    size_t ia, cvm::rvector* __restrict__ const dl0_2_out = nullptr,
-    cvm::vector1d<cvm::rvector>* __restrict__ const dq0_2_out = nullptr,
-    cvm::matrix2d<cvm::rvector>* __restrict__ const ds_2_out = nullptr) const {
+    size_t ia, cvm::rvector* __restrict const dl0_2_out = nullptr,
+    cvm::vector1d<cvm::rvector>* __restrict const dq0_2_out = nullptr,
+    cvm::matrix2d<cvm::rvector>* __restrict const ds_2_out = nullptr) const {
     // if (dl0_2_out == nullptr && dq0_2_out == nullptr) return;
     cvm::real a1x, a1y, a1z;
     // we can get rid of the helper function read_atom_coord if C++17 (constexpr) is available