Framework RHS Splitting and MIS Method

[MarcoArtiano]

This is a first dirty implementation of a general framework for RHS splitting as well as MIS Method.

MIS Method reference
Wensch, J., Knoth, O. & Galant, A. Multirate infinitesimal step methods for atmospheric flow simulation. Bit Numer Math 49, 449–473 (2009). https://doi.org/10.1007/s10543-009-0222-3

The ODE is solved using RK4.

The linear acoustic-advection system has also been added as a test case.

Since many improvements and addition has still to be done, at the moment the PR is still not ready for a review.

ps: I need to implement how to deal with step callbacks when the rhs is splitted.

[github-actions]

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