Gpxity

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* rename Track to GpxFile
* gpxdo clear now does some tasks previously done by gpxdo fix
* gpxdo clear --tracks using new GpxFile.join_tracks()
* .gpxity_config files are also searched in parent directories
* gpxdo fix --unique makes points unique
* Gpx.split_segment_after()
* gpxdo clear --untangle
* gpxdo help, gpxdo help ls
* new backend Memory
* gpxdo with colors
* gpxdo ls --validate-against discovers treating
* gpxity_server is more stable

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* replace auth_cfg with accounts, new syntax for identifiers
* Directory: the file .gpxity_config may define account settings
* WPTrackserver and Directory allow renaming by assigning new id_in_backend
* Backend.subscription shows the name of the subscription model
* gpxdo: dates may now contain a time
* gpxdo split --stops MINUTES instead of fix --jumps
* gpxdo fix --clear-times, --set-first-time, --set-last-time, --add-times, --clear-locations
* gpxdo ls --gap
* new: Track.locate_point() and Track.add_locations()
* split class Account into Account and DirectoryAccount
* integrate ServerDirectory into Directory
* gpxdo fix --add-waypoints adds waypoints to all segment starts

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* fix documentation at readthedocs.io
* gpxdo ls --location shows name of starting place: city, country

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* new backend for Openrunner
* gpxdo --similar now supports a list of tracks
* new: gpxdo --join
* Track.merge (and gpxdo merge) now also merge waypoints
* Up/Download for the same backend class is now lossless for category
* adapted MMT backend to server changes

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* Lifetrack fixes
* gpxity_server now supports https

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* many improvements to gpxity_server and backends with lifetracking capability
* fixes for WPTrackserver (wants local time and speed per position and distance per track)
* new: gpxdo dump --points
* new: Backend.fences for privacy zones