Params (param.py) class needs a tracer_names() method and other classes should use it

[water-e]

There is tracer names functionality in the hotstart generation code. Would like to move this into Params.

Signature probably has to be params.tracer_names(mods=['HYDRO','AGE']) and would take into account schism's order, variables like nage_tracer and the compiled models.

Two milestones: first code the functionality, second is to move it back so it is used by schism hotstart.