Make it model agnostic

[ocefpaf]

For now it is specific to ROMS.

[rsignell-usgs]

Yeah, I guess we can look forward to --roms, --fvcom, --selfe, etc.

[ocefpaf]

This is something we need to think before re-factoring. One idea is to ship templates for roms, fvcom, selfe, etc and load them using a mandatory argument like --model=<model_arg>.

Ee can treat the current implementation as a roms template, factor it out, and providing a model for future templates.

[ocefpaf]

Instead of --model_name we can do that via templates. See #31.