Add in a system identification example

[nic-barbara]

Section VII of the original REN paper includes an example on stable nonlinear system identification with RENs. An old version of the code used to generate these results can be found here.

We should tidy up the code and add this example to the repo. Steps are as follows:

• Create a full working example script system_id.jl and add it to the examples/src/ folder of the repo
• Document and write the example as a tutorial on system identification with RENs just like the other examples in docs/src/examples/