fix: force 'units' kwarg to 'kilometers' in Valhalla router

[mthh]

Following #139, we shouldn't allow the user to pass a “units” argument as a kwarg for the directions and matrix methods of the Valhalla router.

If users use a units kwarg, perhaps it would be a good idea to warn them with a warning saying that their units argument has not been taken into account?
@nilsnolde what do you think?

If we ever support kwargs for OpenRouteService (cf. #42) , we should do the same thing.

[nilsnolde]

I didn't really follow up on the other PR, but yeah, I'd also argue to remove all support for units for any router (if that's the argument). It's too trivial for users to convert themselves. And anyways, fuck imperial units haha they're straight outta hell.

[mthh]

I didn't really follow up on the other PR, but yeah, I'd also argue to remove all support for units for any router (if that's the argument). It's too trivial for users to convert themselves. And anyways, fuck imperial units haha they're straight outta hell.

Indeed, that is already the case (#139 removes unit conversion logic from the various routers concerned, makes explicit the fact that Duration.distance is always in meter, and adds methods to Direction to convert to miles / kilometers).

But still, since we dont control what users pass as kwargs to the relevant routers (listed in #42), they can break the constraint “Direction.distance is always in meters” without routingpy noticing.
So this PR is about preventing the user from passing units=“mi” as a kwarg for Valhalla (which would break the assumption that Direction.distance stores the value in meters).