Move check for boundary conditions type

This previously was checked on every single time step. Now it is checked
once at the beginning in the `Check_Configuration` function.

src/grid/boundary_conditions.cpp

@@ -151,15 +151,6 @@ int Grid3D::Check_Custom_Boundary(int *flags, struct parameters P)
   }
 
   for (int i = 0; i < 6; i++) {
-    /* Alwin: I am disabling this check because it is needlessly occurring every timestep.
-    if (flags[i] < 1 or flags[i] > 5) {
-      chprintf(
-          "WARNING: Possibly invalid boundary conditions for direction: %d flag: %d. Must select between 1 (periodic), "
-          "2 "
-          "(reflective), 3 (transmissive), 4 (custom), 5 (mpi).\n",
-          i, flags[i]);
-    }
-    */
     if (flags[i] == 4) {
       /*custom boundaries*/
       return 1;

src/utils/error_handling.cpp

@@ -54,6 +54,20 @@ void Check_Configuration(parameters const &P)
   #error "Only one integrator can be enabled at a time."
 #endif  // Only one integrator check
 
+  // Check the boundary conditions
+  auto Check_Boundary = [](int const &boundary) {
+    bool is_allowed_bc = boundary >= 0 and boundary <= 4;
+    assert(is_allowed_bc &&
+           "WARNING: Possibly invalid boundary conditions for direction: %d flag: %d. Must select between (periodic), "
+           "2 (reflective), 3 (transmissive), 4 (custom), 5 (mpi).\n");
+  };
+  Check_Boundary(P.xl_bcnd);
+  Check_Boundary(P.xu_bcnd);
+  Check_Boundary(P.yl_bcnd);
+  Check_Boundary(P.yu_bcnd);
+  Check_Boundary(P.zl_bcnd);
+  Check_Boundary(P.zu_bcnd);
+
   // warn if error checking is disabled
 #ifndef CUDA_ERROR_CHECK
   #warning "CUDA error checking is disabled. Enable it with the CUDA_ERROR_CHECK macro"