Add "state space" / dynamical systems examples

[sjdaines]

Add examples to illustrate use of PALEO in more general situations than just solving an ODE and calculating a time series.

The closest relevant example at the moment is https://paleotoolkit.github.io/PALEOtutorials.jl/dev/collated_examples/solvers/README/#Naive-first-order-explicit-Euler-'by-hand' which shows how to calculate a time derivative and update state variables to step through a solution.

Need to add examples that start by demonstrating the general approach:

1. set state variables
2. call PB.do_deriv(modeldata.dispatchlists_all) to update all model variables for this state (these are held in the modeldata object)
3. read out whatever quantities are of interest using eg PALEOmodel.get_array(modeldata, "ocean.P_norm").values