FFTW grid sizes

Description

Do not allow FFTW grids that are not compatible with CUFFT ones. This avoids getting different results between GPUs and CPUs.

News snippet

Do not allow FFTW grids that are not compatible with CUFFT ones.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

src/math/fftw_low.c

@@ -28,6 +28,9 @@
  * where a,b,c,d are arbitrary and e,f are 0 or 1.
  * (http://www.fftw.org/doc/Complex-DFTs.html)
  * par is the parity: the result must satisfy n % 2 == par, provided par >= 0.
+ *
+ * NTD: To make cufft and FFTW to always give the same results, the multiples
+ * of 11 and 13 are not allowed.
  */
 void fft_optimize(int *n, int par)
 {
@@ -46,9 +49,9 @@ void fft_optimize(int *n, int par)
       if(n2 % i == 0){
         /* For debugging:    */
 	/* printf("%i ", i); */
-	if(i > 13) break;
+	if(i > 7) break;
 	n2 = n2 / i;
-	if(i != 11 && i != 13) i--;
+	i--;
       }
     }
     /* For debugging: */