dsl: Introduce abstractions for multi-stage time integrators

[fernanvr]

Implementation of the ideas of @mloubout, as discussed in the Slack channel #timestepping.

It were created two new classes:

• MultiStage: Each instance represents a combination of a PDE and its associated time integrator.

• RK: Instances encapsulate a Butcher Tableau. The class methods define specific Runge-Kutta schemes, and it includes logic to expand a single PDE into multiple stage equations according to the RK method. It only contains the classic RK44, of fourth order and four stages, as a prove of concept.

To integrate the MultiStage into Devito’s pipeline, we modified the _lower(...) function in operator.py by adding the line:
expressions = cls._lower_multistage(expressions, **kwargs),
and modified _sanitize_exprs(cls, expressions, **kwargs) to recognize MultiStage instances.

We also created a new function:
_lower_multistage(),
in the same file. This function handles the expansion of MultiStage instances into their corresponding stage equations.