Add a parameter to control the percentage used in get_prob_by_od_and_mode

[Mind-the-Cap]

In get_prob_by_od_and_mode we use a parameter that's currently fixed.
The default value, 99.9 %, seems to do the job but it could be interesting to see if lower values improve speed and the impact on precision.

#289 (comment)

[FlxPo]

This method is actually not used anymore, because modal choice is now a consequence of the full plan choice. We should probably remove it now ?