Overhauled Gateway Code

* Split off schematic-related functionality into BaseSchematicGateway.
* Moved the default implementations of many methods to the base classes
because having a bunch of duplicate stubs all over the place was a
waste.
* Removed methods that were redundant or weren't being used for
anything.
* Added support for importing schematics while ignoring air blocks
through IBlockSetter - now our gateways are copied in without air by
default.
* Fixed bugs that would have prevented the sandstone gateway from
generating.
* Removed the code in generate() that would cause dungeon
pre-generation. We should solve this by attaching data to links instead
and it's not a feature that we're even using right now (everything uses
the default pack).
* Fully documented our functions - it's so beautiful...