[DOC] Format acquire's reference documentation
[DOC] Reformat viktrack's reference documentation
[DOC] Improve babel's reference documentation
Simplify acquire feature

Remove type member from VikDataSourceInterface and use only process function.

Signed-off-by: Guilhem Bonnefille <guilhem.bonnefille@gmail.com>
[DOC] Improve aquire's reference documentation
Allow to download and acquire a GPX without gpsbabel

Like a_babel_convert_from_shellcommand, when input_file_type is NULL
gpsbabel is not used. Instead, the file is read just after download.
Avoid gpsbabel conversion on OSM traces

Disable the gpsbabel conversion as retrieved data is already GPX.