Coupling techniques

Particle-fluid coupling, unresolved:

Topic	Notes
Test cases/ Benchmarks	Single particle sedimenting
References	openFOAM-Liggggths unresolved particle-fluid coupling [link, benchmarks] EDEM-Fluent coupling [link] Monolithic oomphlib fluid-structure coupling [link] oomph-lib resolved particle-fluid interaction [link] fully resolved p-f [link]
openFOAM-Liggggths unresolved, incompressible particle-fluid coupling	volume-averaged Navier-Stokes Piso pressure-velocity coupling for Navier-stokes Benchmark: two particles kissing and sedimenting dt_Particle << dt_Fluid