Rkm-codes

- Suppress *rkm_code*’s use of the new SI scale factors in [QuantiPhy](https://quantiphy.readthedocs.io) so that ``R`` is no longer treated as a scale factor when using the latest version of *QuantiPhy*.

- Allow argument to *to_rkm()* to be a string or simple number
- Added *strip_code* preference
- With small numbers show 0 rather than exponent

- fixed issue in *set_prefs()*.

Initial release