Leftover ideas for ODEs

Leftover ideas after #985

# TODO

Some Runge-Kutta schemes have the First-Same-As-Last (FSAL) property, that enables sharing a residual evaluation from one step to the next. It may not be possible to have this optimisation in our case because of the Dirichlet boundary conditions.

# Ideas for later

* Adaptive time-stepping with embedded Runge-Kutta methods or Richardson extrapolation
* Linear multistep methods (Adam-Bashford, Adam-Moulton, Backward Difference Formula)
* General linear methods
* Numerical methods for differential-algebraic systems of equations (DAEs)