defaults on exclusive numeric attributes like height, weight

I'm unsure if we're handling numeric defaults for exclusive attributes like `weight`/`height` etc really well.

e.g. `truck` has really high defaults:
https://github.com/valhalla/valhalla/blob/5939f8fd32b57ddfe57cd1379d57ae8db8e47c24/src/sif/truckcost.cc#L50-L54

it might sorta reflect the average truck, but 12 tons and less than 4 m height can also be considered a truck (in Germany at least). shouldn't we rather default to 0 for these kind of exclusive attributes instead of trying to guess? if people think they need those attributes for their routes, I think they really should explicitly specify their dimensions etc.

opinions?