v0.7.6 of activity_files

A pure Dart toolkit for parsing, editing, validating, and converting workout activity files in GPX, TCX, FIT, GeoJSON, and CSV formats. Changelog excerpt: ### Fixed - GPX trackpoints missing ` `no longer drag the segment's lap bounds to the epoch fallback. - TCX no longer drops a lap's first trackpoint when `StartTime`postdates it. - `validateLapBoundaries()`no longer skips corruption checks on…