Fix various bugs when using custom units

Minor bug fix

Fix local elevation (ground altitude = 0) using minimum recorded altitude instead of online API.

* Customize your desired units to mix metric and imperial standards. Check the settings menu. * Improved chart rendering by filtering out dive angle and Glide ratio after landing. Full list of updates available here: https://github.com/vef445/AirtimeBT

* Swoop phase autodetection: whether you're doing 90s, 270s, 450s or world competition level swoops, you can now choose to automatically cut the track and only keep the swoop phase for analysis. Hit the settings button and select your favorite auto cut (never, from exit to landing, swoop). * Exit point detection improvements to exclude false positives * Minor UI improvements and bug fixes

* New Speed Run Analysis, incl. exit altitude, validation and scoring windows, tracking errors statistics * Local elevation is now based on open-elevation.com API and retrieved automatically (no more funky calculations) * Auto detection of your exit and landing points (can be deactivated in the settings) * New Help pane. Check it out via the "?" button * UI changes for more clarity and focus on what matters * Multiple bug fixes

* Assess your wingsuit flight performance with the new speed polar chart * Speed skydivers: Airtime BT now displays your Fastest Freefall Average Speed over 3sec

* You can now share your track.csv or save it locally, all from the new Share button. * Complete review of the UI for more clarity: press and hold the values to see their corresponding unit (space saving), burger menu to display available actions and submenus (space saving)