[ENH] Better representative time calculation for mixed condition/wet race

[Casper-Guo]

Proposed new feature or change:

The changes implemented in #113 does not populate the deltatorep and pctfromrep columns because the representative time is not calculated in the case of Sao Paulo GP, where all laps are ran on wet tyres.

This is not the optimal experience for the dashboard. Try building in more tolerance for non-dry races and make sure the median times can always be reasonably calculated.