Remove journey with more fallback than the matching non_pt 

For exemple with the walking, if the non_pt_walk has a duration of 12
minutes, the following journey will be masked:
    - walking 14min + TC
    - walking 8min + TC + walking 6min

refer to: http://jira.canaltp.fr/browse/NAVITIAII-1304
isn't it clearer to compare it with func(type_journeys[type]) ?

I know it is the same thing but it would be easier to read AFAIK.

And a bit of comment on this function wouldn't hurt.