CreateTiledGround() function 

I just added the function in Mesh and VertexData class of js files. I don't really know how typescript works with "js-style" objects, and this is why I did'nt change typescript files.

I think it would need also changes in the Geometry class ? I am not sure and I don't want to make mistakes.

### About the use of the function 
It takes 6 arguments : 
* *xmin, zmin, xmax, zmax* : extent values of the ground
* *precision* : the number of subdivisions inside a tile.
* *subdivisions* : the number of tiles