Extend and document createTilesetJson

[javagl]

Fixes #104

The createTilesetJson function was not documented in the main README.md. This is now added here.

Additionally, the command is extended to receive a position for the generated tileset. The position is passed as cartographicPositionDegrees to the command, which can be a 2- or 3-element array that contains the longitudeDeg, latitudeDeg, <heightMeters> that should be used for the root transform of the generated tileset. For example:

npx ts-node ./src/cli/main.ts createTilesetJson -i ./temp/ -o ./temp/tileset.json --cartographicPositionDegrees 
-75.152 39.94 123.0

I hesitated a bit with the exact format. Passing around an number[] array feels a bit awkward, because you never know its length. Alternatives like --longitudeDegrees 75 --latitudeDegrees 39 --heightMeters 123 would be pretty inconvenient, and there's no way to model the constraint that the first two parameters are required (together), and the last one is optional. So I think that the current solution is a reasonable middle ground...