merge_struct_arrays: add keep_null qualifier

* Helpful in combination with get_params if some parameters are tied
  and others are not
* Without keep_null qualifier merge_struct_arrays(get_params())
  filters the NULL entries in the tied field, (i.e., all untied
  parameters are filtered out), which causes problems if one wants to
  filter the struct later.

src/slang/structs/tables/struct-arrays/merge_struct_arrays.sl

@@ -9,7 +9,8 @@ define merge_struct_arrays()
 %    All elements of s have to be structures with the same fields.
 %    The return value merged_s is another structure of this kind, and
 %    merged_s.field = [s[0].field, s[1].field, ..., s[-1].field];
-%    holds for every field. (If s[i].field is NULL, it is skipped.)
+%    holds for every field. (If s[i].field is NULL, it is skipped
+%    unless "keep_null" qualifier is set.)
 %\qualifiers{
 %\qualifier{remove_excess_fields}{: remove fields not present in all
 %       structures.}
@@ -17,6 +18,7 @@ define merge_struct_arrays()
 %       dimensions of the original fields, using the sum of dimensiom 'dim'
 %       to account for the increased array. Care has to be taken that the
 %       other dimension need to have the same length in all structures. }
+%\qualifier{keep_null}{: keep fields which contain NULL.}
 %}
 %\seealso{append_struct_arrays, get_intersection, reshape}
 %!%-
@@ -50,13 +52,13 @@ define merge_struct_arrays()
   {
     variable  findim = 0  ;
     variable a = get_struct_field(s[0], field);
-    if(a!=NULL){  a = [a];
+    if(a!=NULL or qualifier_exists("keep_null")){  a = [a];
      (dim,,)= array_info(a) ;
     findim += dim[resh] ; 
     }
     _for i (1, length(s)-1, 1)
     { variable a_ = get_struct_field(s[i], field);
-      if(a_!=NULL) { a = [a, a_];
+      if(a_!=NULL or qualifier_exists("keep_null")) { a = [a, a_];
 	(dim,,)= array_info(a_) ;
 	findim += dim[resh] ;
       }