Add nan value handling in flatfield.cu

nabu/cuda/src/flatfield.cu

@@ -65,5 +65,11 @@ __global__ void flatfield_normalization(
         #error "N_DARKS > 1 is not supported yet"
     #endif
 
-    radios[pos] = (radios[pos] - dark_val) / (flat_val - dark_val);
+    float val = (radios[pos] - dark_val) / (flat_val - dark_val);;
+
+    #ifdef NAN_VALUE
+    if (flat_val == dark_val) val = NAN_VALUE;
+    #endif
+
+    radios[pos] = val
 }