Version 3.7.0: Numerous usability enhancements

What's Changed

• timescale.rs: derive serdes traits when feasible by @gwbres in #167
• timecale.rs: introduce format/display by @gwbres in #168
• readme: fix BeiDou typo by @gwbres in #169
• epoch: derive Hash by @gwbres in #170
• timescale: identify GNSS timescales from standard 3 letter codes by @gwbres in #171
• timescale: standard formatting is now available by @gwbres in #174
• epoch, duration: improve and fix serdes feature by @gwbres in #175
• epoch, timescale: implement default trait by @gwbres in #176

Full Changelog: 3.6.0...3.7.0