Fix benches/test

Author: Keats

benches/benchmark.rs

@@ -13,6 +13,7 @@ fn build_bfield(n_secondaries: u8) -> BField<String> {
         0.1,
         0.025,
         n_secondaries,
+        false,
         String::new(),
     )
     .expect("to build")

src/bfield.rs

@@ -293,7 +293,7 @@ mod tests {
             let val = bfield.get(&i.to_be_bytes().to_vec()).unwrap();
             assert_eq!(i, val);
         }
-        bfield.persist_to_disk().unwrap();
+        bfield = bfield.persist_to_disk().unwrap();
         for m in &bfield.members {
             assert!(m.filename.exists());
         }