[Feature] Blood, vessels and heart simulations

[lhemerly]

Introduce multiphase support for SPH-based particle entities to enable realistic blood simulations. This enhancement lays the groundwork for future developments in blood, vessel, and heart simulations.

Implements #373

[lhemerly]

Thanks for the hint on creating a Solver, it makes more sense now that I understand better the codebase.

This is a first draft to align how we would be making it. I did not write tests neither tested in the viewer yet.

A few things to think about:
1 - Should the solver be a general multiphase solver with blood being inherited from that general multiphase solver as an specialized solver?
2 - How do we integrate it with the coupler? Would we need a specialized solver for FSI with multiphase?
3 - Is the file structure fine? Maybe a baseline of solvers with some kind of "plugin" folder for specific implementations?

There's still several TODOs for a full implementation:

1 - Review of the current implementation (baseline for multiphase/blood)
2 - Coupler FSI integration with multiphase SPH, shear rates, yield and mainly with the blood constituents changing shape and biochemical
3 - Pulsatile flow emitter implementation

It's worth nothing that there could be severeal different implementations of this with different levels of accuracy and compute cost. We could also implement a simplified version.