Make downloaded CUP file more robust

[mathias-muench]

Describe the Feature

In CUP file downloaded from WeGlide task, repeat first turnpoint in "takeoff" and last turnpoint in "landing".

Right now, takeoff and landing are empty (""). While SeeYou itself is quite happy with that, it leads to strange behavior in third-party applications. XCSoar skips the last turnpoint (see WeGlide task cup file not compatible with XCSoar · Issue #91 · weglide/bugtracker), Java TopTask repeats random other turnpoints.

According to SeeYou CUP file format description - SeeYou_CUP_file_format.pdf (2022), "takeoff" and "landing" are still part of the file format, although different applications obviously handle them differently. Repeating the first turnpoint in "takeoff" and last turnpoint in "landing" seems to make all systems happy that I am aware of.

Regardless of a discussion about "officially correct behavior", it would make things more robust, even for legacy systems.

Example

Current task line:

"744 km · Dreieck · DMSt · Unterwoessen Kirche","","Unterwoessen Kirche","Mariazell Tennisplatz","Flossenbuerg Burgruine","Unterwoessen Kirche",""

Proposed task line:

"744 km · Dreieck · DMSt · Unterwoessen Kirche","Unterwoessen Kirche","Unterwoessen Kirche","Mariazell Tennisplatz","Flossenbuerg Burgruine","Unterwoessen Kirche","Unterwoessen Kirche"