message: Use prelude to load DataFrame
diff: @@ -20,7 +20,7 @@ pub use errors::{Arrow2DestinationError, Result};
  use fehler::throw;
  use fehler::throws;
  use funcs::{FFinishBuilder, FNewBuilder, FNewField};
  -use polars::frame::DataFrame;
  +use polars::prelude::DataFrame;
  use std::convert::TryFrom;
  use std::sync::{Arc, Mutex};
  pub use typesystem::Arrow2TypeSystem;
  