Rework default interface for Problem class

[tlunet]

Following discussion #253, set a more generic interface for the problem class for more generic ODE, with a default implementation considering the simple generic ODE system

$$\frac{dU}{dt} = f(U,t)$$

Add full documentation in the Problem class.

⚠️ this should probably be done once #244 is merged into v6, and before going through the associated rework of all problems implementations (#255).