Rename stronghold to mcutil to facilitate additions.

This will be used to consolidate some Python files sitting around
my storage media.
Add mcutil.nether: for converting coords.
Rework the types (classes) to distinguish between Vector and Point.

Note that in Point3D z and y are reversed. This is correct. The
intent is to allow the usage of a Point and a Point3D in operations
that do not utilize the y component. While it many be intuitive,
it greatly simplifies Point* usage.