Make Z axis land level default n number of meters above zero bound

[EmmaMakhno]

Make the default land level some number of meters up on the z axis

This will allow transport infrastructure to be developed under peoples parcels, it will make it easier than later having them redo stuff.

Lower layers could be ceded to voluntary cooperative regional transport authorities without having cost to Land owner because the z axis is infinite. Any costs at the p2p layer could be paid by transport authority through various means. Since the z axis is unlimited and it wont impose on the usability of the Land owners parcel it should be easy to gain cooperation from majority of Land owners.

[pscott]

Not sure if it has been specified yet, but maybe this won't be needed if we can dig "below" (i.e if negative value for the z axis are possible).