Add to_float timestamp methods and further support to negative timestamps

[philipnbbc]

Details

This PR

• adds Timestamp.to_float and Timestamp.to_unix_float methods
• removes deprecated Timestamp.to_utc
  • Only the Squirrel garbage collector layer uses this method and it is updated in https://github.com/bbc/rd-cloudfit-squirrel-media-store/pull/513
• Changed Timestamp.to_unix to return the sign as well in the tuple result
• supports conversion between negative Timestamp and datetime and iso8601
• fixes Timestamp.from_unix when is_leap is True

Pivotal Story

Story URL: https://www.pivotaltracker.com/story/show/187046592

Related PRs

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• Any temporary code/configuration removed (e.g. test deployment environment, temporary commontooling branch)
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[samdbmg]

LGTM