Support zero-second split times

[LukeWoodward]

Robert Marique from Helga-O has requested that SplitsBrowser supports zero-second split times. These will typically arise from an event with a road crossing, where competitors have a maximum amount of time to cross a road and the splits are later modified to remove the time spent waiting at the road crossing.

Presently SplitsBrowser doesn't permit zero-second splits: it regards these as dubious and wipes them out.

There will be some knock-ons to this change. In particular, the percent-behind graph can't show a meaningful point if the reference split for a control is zero, but on the other hand a zero reference split means that the x-coordinate for that control after is the same as that for the control before, so we can use the previous percent-behind value.

Time-loss calculations will also need to be adjusted to exclude zero-second split times.

[LukeWoodward]

Following discussion with Robert at Helga-O, zero-second splits will now be permitted provided a new top-level option permitZeroSplits exists and is set to true.