Terrain factory implemented in WorldEnv's TerrainMixin

[mbasel-t]

Added terrain functionality to WorldEnv. With how defaults are implemented, current code using WorldEnv should not be bugged, as its functionality aside from terrain configs is untouched. I put the factory in gym/terrain/terrain.py because the code is too hefty to be put in a file in the mixins folder in my opinion.

I also created a short testing program that creates a simple Genesis simulation using WorldEnv, which should be kept in the repository because WorldEnv is an important part of the program which should be continually maintained, and because it serves as a reference for using WorldEnv to create a simple environment in future testing and training. The testing program is not complex at all, and one could definitely argue in-favor of a more complex test, but the test in its current state at least serves as a foundation for a more complex test to be created at a later date.