Last touches to fix up #1563:

1) On GPU:
   'NULL' (automatic) mode is mapped to 'default' which means 'regular'
2) When explicitly using 'regular' (or arriving there by logic on GPU)
   terminate in error if dataset axie-dimensions are not equal.

common/lib/share/interpolation-lib.c

@@ -374,7 +374,7 @@ struct interpolator_struct *interpolator_load(char *filename,
 
  #ifdef OPENACC
  if (method && strlen(method) && (!strcmp(method, "kdtree"))) {
- fprintf(stderr, "interpolator_load: FATAL ERROR: 'kdtree' is not supported on OpenACC/GPU - only 'regular' works!\n");
+ fprintf(stderr, "\n\n!! interpolator_load: FATAL ERROR: !! \n'kdtree' is not supported on OpenACC/GPU - only 'regular' works!\n\n");
  Table_Free(&table);
  exit(-1);
  }
@@ -468,10 +468,21 @@ struct interpolator_struct *interpolator_load(char *filename,
  /* assign interpolation technique: 'regular' direct indexing */
  if (!strcmp(interpolator->method, "regular")) {
  interpolator->kdtree = NULL;
+ /* Terminate if the dimensions are not equal */
+ if (interpolator->bin[0] != interpolator->bin[1] ||
+ interpolator->bin[0] != interpolator->bin[2] ||
+ interpolator->bin[1] != interpolator->bin[2]) {
+ fprintf(stderr,"\n\n%s\n\n",
+ "!! interpolation-lib ERROR: !!\n"
+ " You are running the 'regular' interpolation scheme\n"
+ " with a file of unequal axis dimensions. This will lead to systematic errors!\n"
+ " Please either resample the file to achieve this or run with 'kdtree'\n"
+ " (kdtree is available on CPU only)");
+ exit(-1);
+ }
  /* store table values onto the grid: each field component is stored on the
  * interpolator->grid, and has size=prod(interpolator->bin)
  */
-
  long prod=1; /* the number of elements in the grid */
  for (dim=0; dim<interpolator->space_dimensionality; dim++)
  prod *= interpolator->bin[dim];

mcstas-comps/optics/Pol_tabled_field.comp

@@ -158,6 +158,16 @@ INITIALIZE
  /* these default field functions are part of pol-lib */
  /*initialize the interpolation vertex structure*/
 
+
+ #ifdef OPENACC
+ // If "aotomatic mode is given on GPU, switch
+ // to 'default'
+ if (!strcmp(interpol_method,"NULL")) {
+ sprintf(interpol_method,"default");
+ }
+ #endif
+
+ // Handle default mode in both CPU and GPU case
  if (!strcmp(interpol_method,"default")) {
  #ifdef OPENACC
  // Default is to assume regular interpolation on GPU
@@ -168,7 +178,7 @@ INITIALIZE
  sprintf(interpol_method,"NULL");
  #endif
  }
- 
+
  interpolator = interpolator_load(filename, 3, 3, interpol_method);
  interpolator_info(interpolator);