[Do not merge yet] Add 'Mizuki', an SPH/Nbody code based on FDPS

[rieder]

Very simple C++ SPH+Nbody code, based on the nbody+sph c++ example of FDPS. Meant to be used as a 'kick' code / field code, not as much as an integrator (though it can and does work as one). Not finished yet so don't merge yet. Continuation/rename of #812