Converter for Rohde & Schwarz Recordings

[Teque5]

Originally discussed in #23, we want to implement a converter for recordings made by Rohde & Schwarz RF recorders.

• Use CLI sigmf_convert entry point and use sigmf.convert.rs.rs_to_sigmf() as entry point.
• Use sigmf.fromfile() entry point and use magic bytes b'R&S ' detection method.
• Support --archive and --ncd modes if possible
• Use example recordings in nonsigmf examples repo in test_convert_rs.py
• Add to converters.rst in Documentation