Add `SetOfNumericalSchemes._automatic_plan`

[nobodyinperson]

A SetOfNumericalSchemes knows everything it needs to know to automatically determine the correct equation solving order:

• every equation's dependencies (Equation.input)
• every numerical scheme's needed timesteps (NumericalScheme._needed_timesteps)

The method that needs to be implemented is SetOfNumericalSchemes._automatic_plan.