Introduce base environment class

[smarianimore]

The single and multi agent environments actually share a great deal of their source code, as it is basically the implementation of a grid world.
However, they cannot collapse to a single class as they are meant to be compatible with Gymnasium (single agent) and PettingZoo (multi-agent).
Nevertheless, a base class could be introduced to deal with much of the code, to be "wrapped" inside two "facade" classes, one for Gymnasium and one for PettingZoo, simply adapting the interface and provided the needed additional stuff