feat(iox): add iox.sqlInterval to convert flux durations to SQL interval strings

[sanderson]

This PR updates the iox package with examples and a new iox.sqlnterval function that converts a Flux duration value to a SQL interval string (2d12h -> 2 days 12 hours). This stems from a request to be able to use v.windowPeriod with iox.sql to define window boundaries.

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