make normalize_value more efficient

src/subroutines/normalize_value.m

@@ -31,7 +31,7 @@ function series = normalize_value(series)
 
 if iscell(series.value)
     series.original_value = series.value;
-    series.value(cell2mat(cellfun(@(x)strcmp(x,'NA'),series.value,'UniformOutput',false))) = {NaN};
+    series.value(strcmp(series.value, 'NA')) = {NaN};
 else
     series.original_value = num2cell(series.value);
     series.value = num2cell(series.value);