[simulator][stable fluid](up to 10 pts) Fluid-rigid coupling (variational fluid)

**Prerequisites**: stable fluid solver (#24), rigid body dynamics (#20)

# Variational Fluid

Implement variational fluid solver coupling with single rigid body (4 pts)
Implement variational fluid solver coupling with articulated rigid body (6 pts)

## References

[Variational Fluid](https://dl.acm.org/doi/abs/10.1145/1276377.1276502)