Proper resources for prospecting

[domob1812]

When a region is prospected, we should randomly select one type of resource that can be found there, and randomly determine how much there is. Both may depend on the physical coordinate of the region. Then this is stored together with the block height when it was prospected in the region data (replacing the current storage of an "owner").

Also, after 5,000 blocks (or some other length of time) has passed and all resources are gone, it can be prospected again.

Prizes will work as items instead of being tied to regions, as described in #49.