Add support for ROMS

[MaceKuailv]

It should be relatively straightforward to adapt to ROMS dataset. This is a bite-size goal to address #78.

Here is an example flow field from our friendly JOSS reviewer Pål Næverlid Sævik: https://github.com/pnsaevik/ladim_plugins/blob/master/ladim_plugins/chemicals/forcing.nc

[ThomasHaine]

Good idea!

[MaceKuailv]

Tarun has a little POP model, I am going to look at that as well.