a bug fix

Works around a bug in either Rust or Rust bzip2 bindings whereby an unused but initialised bzip2 streaming decompressor could not be dropped in a clean manner. As a result a return from a Rust function would hang indefinitely. A workaround uses Option<Rc<RefCell<bzip2::write::BzDecoder<Vec<u8>>>>> etc.

The problem affected the JVO server version of FITSWebQL.