Update finalize to meet standard

[shuds13]

• Remove finalize return and argument
• Add get/export_data function.
• Ensure finalize finishes any comms with remote process (and shuts down) so export simply returns data.
• Add user_fields option to export to return H with user fields (e.g., beam_length)
• Add tests for new unmapping function + finalize/export.

[jlnav]

Good catch. Thankfully I believe I adjusted the runner recently to not suggest/ingest data from finalize anymore,

        tag, Work, H_in = self.ps.send_recv(H_out)  # evaluate the initial sample
        final_H_out = self._start_generator_loop(tag, Work, H_in)
        self.gen.finalize()

but the signature obviously was still incorrect