Add tutorials

[tjstienstra]

This is an issue tracker, that follows up on the preliminary merged PR #111.

Two tutorials should be added:

• [x] The third tutorial, which is the tutorial for model developers to create their own models, connections, and load groups. This can follow Appendix C from my thesis, where the rolling disc is implemented from scratch.
• [x] Either a separate tutorial or an addition to the second BRiM users tutorial, such that it is more clear how to do on-the-fly customizations of BRiM models.

[tjstienstra]

The second bullet point is partially in the first second tutorial, so I will leave it out for now.