Add `.TCX` download with turn by turn navigation

[maxibor]

Nice work on the new openrouteservice web frontend looks very nice, and renders very nice on mobile compared the classic one 😃

One thing that was added last year in the classic client, thanks to the work of @TheGreatRefrigerator and @marq24 (#323), was the possibility to download in .tcx format which is used for turn by turn navigation in handheld GPS unit (bike computers for example).

Here is an exemple of a .tcx file generated with the classic ORS frontend

ors-export-linestring.tcx

Could it be possible to port it as well in the new web frontend ?

I see that for .gpx you use the togpx package of @tyrasd , maybe can be adapted to .tcx ?

[marq24]

I'll have to check the new web frontend - but in general it should not be a big issue to port the tcx impl... at least right now I don't see a show stopper