Add "fluid-structure interaction" example

[briandrawert]

Original code: https://github.com/briandrawert/SPH-BVF/
Fix Mechanics: https://github.com/briandrawert/SPH-BVF/blob/master/src/USER-SSA-TSDPD/fix_ssa_tsdpd_bvf_mechanics.cpp
Pair Style Mechanics: https://github.com/briandrawert/SPH-BVF/blob/master/src/USER-SSA-TSDPD/pair_ssa_tsdpd_bvf_mechanics.cpp