Generic tracer module

We provide this module to users as template for potential addition of behavior etc. As is, the module simulates multiple classes of passive tracers with constant settling velocity gen_wsett (gen_wsett <0 =>swimming velocity), with no body forces and zero fluxes at surface and bottom. The number of generic tracers is specified as ntracer_gen. I.C. flag is flag_ic(3), and nudging flag is inu_tr(3) in param.nml. The output flags are iof_gen(1:ntracer_gen).