fix serialization

src/gridcounts.rs

@@ -258,7 +258,7 @@ impl GridCounts {
         self.counts = counts;
         self.shape = shape;
         self.resolution = resolution;
-        self.set_n_threads(n_threads)?;
+        self.set_n_threads(Some(n_threads))?;
 
         Ok(())
     }