mesa-2.x legacy branch for Mesa 2.x examples

[EwoutH]

We've created a mesa-2.x branch for Mesa 2.x examples, while we continue development towards Mesa 3.0.

For new pull requests:

• To update/add an example for Mesa 3.x, target the main branch (default)
• To update/add an example for Mesa 2.x, target the mesa-2.x branch (default)

We very much welcome new Mesa 3.0 features to be added to the examples, which should now be easier targeting only Mesa 3.0. See:

• Use new AgentSet functionality in examples #111
• Use new Cell Space functionality in examples #123
• Use new Discrete Event Scheduling in examples #124