Procedural Generation Library Updates (#217)

* Procedural Generation library has been renamed to `mcg` and builds into the static library `libmcg`
* Library source code has been streamlined to include only the core classes and an example. 
* Streamlined library code lives in `libs/mcg/`
* Library example corresponds to the Tutorial section of the paper and lives in `libs/mcg/examples/`
* Visualiser is now called `mcgviz`
* `libs/mcg/` also includes a new `README.md` with library usage instructions
* Library algorithms specific to ToMCAT have been moved to `exe/mcgen/`
* Added a script `run_mcg_tutorial` to `tools` for creating and viewing the library's tutorial world
* The `Pos`, `AABB`, `Block`, `Object` and `Entity` classes have been updated with one or more new shift methods. Shifting `AABBs` does clear the `AABB` instance's `Connection` list due to the unreliable nature of shifting a connection
* CMakeLists.txt updated to reflect new library structure
* Library documentation updated
* Planning and plan recognition code moved to ml4ai/tomcat-planrec