task: Use aerofiles.xcsoar.Writer in xcsoar_writer.py
task: Use aerofiles.seeyou.Writer in seeyou_writer.py
task: Moved unrelated code out of the with clause
task: Removed dead code path
task: Merged duplicate code
task: Moved more code out of unrelated with clauses

.. including dead code path cleanup
task: Simplified if-return statements
task: Extracted write_xcsoar/seeyou_task() functions
task: Use write_xcsoar/seeyou_task() functions instead of create_ counterparts
task: Removed obsolete create_xcsoar/seeyou_task() functions
task: Renamed "key" enumeration variables to "i"
task: Simplified write_xcsoar_task() function
task: Simplified write_seeyou_task() function
Merge turbo/aerofiles branch