Added skeletal Sector and Wall classes.
Very beginnings of worldbuilding.
Added some of the initial wall/sector linkages for world building.
Beginnings of a World Test.
Added some immutable getters for sectors and walls.
Some basic error handling for MapFile.
Added basic map validity tests.
Revert "Added basic map validity tests."

This reverts commit fb2e8e671e94a1a82101b9cb1d919c2535328bde.
Replaced map file valitity checks.
Actual validity checking will have to be done upon world load (a la end to end principle).
Added semantic tests for map validity.
Added Sprite building. Building toward a formalized "link" concept.
Filled out WorldTest and added needed accessors to world elements.