Upgrade should remove the leading slash in tile url

[lilleyse]

If a tile url has a leading slash it should be treated as a warning, if not an error.

See CesiumGS/cesium#6125

[lilleyse]

Also add this fix to the upgrade command in 3d-tiles-tools to help fix existing tilesets.

[javagl]

I'll consider this as a special case of CesiumGS/3d-tiles-validator#208 for the validator part.

So I'll transfer this one to the tools repository, to handle the possible change in the upgrade function.

[javagl]

Right now, the upgrade does not remove the leading slash. But this function could be added trivially.

If there really are tilesets that use a leading slash, one could make a stronger case for removing it: For things like 3TZ, paths are expected to have no leading slash. This is currently ensured (again) in the 3TZ writing layer.

The only concern that I'd have: The specification does not say anything about that. I think that a leading slash should be valid, and would have a hard time justifying why the URI should be modified if it is not necessary...