Add Robinson Crusoe Economy.

[peisenha]

We want to add a small Robinson Crusoe model to our list of example models for use in teaching.

[tobiasraabe]

@peisenha I added the two Robinson Economies as a basic and extended model to the package and more explanation to the parts describing the options and parameters.

I had to remove the check for the number of states in the initial period because it is not clear how to make the check work for the extended rce. One has to infer that friday is only available starting with period 2.

Please have a look at the changes and leave a comment.

[peisenha]

@tobiasraabe , looks great!

@amageh @timmens We will decouple our lectures from the respy example models for now so we can close this PR and move on. I will store the params and options for our models in our respy-lectures tutorial . I will reach out on Slack with the details once done.