Decide on API for defining VectorFields

[VeckoTheGecko]

With #2646 - we need to decide on an API for adding VectorFields.

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I was looking at the docs build, and saw an example that adds a second velocity field

import matplotlib.pyplot as plt
import numpy as np
import xarray as xr

import parcels
import parcels.tutorial

# Load the CopernicusMarine data in the Agulhas region from the example_datasets
ds_fields = parcels.tutorial.open_dataset("CopernicusMarine_data_for_Argo_tutorial/data")
ds_fields.load()  # load the dataset into memory

# Create an idealised wind field and add it to the dataset
tdim, ydim, xdim = (len(ds_fields.time),len(ds_fields.latitude), len(ds_fields.longitude))
ds_fields["UWind"] = xr.DataArray(
    data=np.ones((tdim, ydim, xdim)) * np.sin(ds_fields.latitude.values)[None, :, None],
    coords=[ds_fields.time, ds_fields.latitude, ds_fields.longitude])

ds_fields["VWind"] = xr.DataArray(
    data=np.zeros((tdim, ydim, xdim)),
    coords=[ds_fields.time, ds_fields.latitude, ds_fields.longitude])

fields = {
    "U": ds_fields["uo"],
    "V": ds_fields["vo"],
    "UWind": ds_fields["UWind"],
    "VWind": ds_fields["VWind"],
}
ds_fset = parcels.convert.copernicusmarine_to_sgrid(fields=fields)
fieldset = parcels.FieldSet.from_sgrid_conventions(ds_fset)

# Create a vecorfield for the wind
windvector = parcels.VectorField(
    "Wind",
    fieldset.UWind,
    fieldset.VWind,
    interp_method=parcels.interpolators.XLinear_Velocity
)
fieldset.add_field(windvector)

We no longer have add_field - I think we need to map out exactly how we should be defining velocity fields under this new refactoring... I think that can be done in a future PR

Originally posted by @VeckoTheGecko in #2646 (comment)

[erikvansebille]

Is the solution here to create two field sets (one for currents and one for wind), and then add the FieldSets? Or wouldn't that work because they all come from the same ModelDataset?

[VeckoTheGecko]

It wouldn't work since VectorFields (called UV and UVW) are constructed from U and V fields in both cases - this is hardcoded. Adding them together will (by design) error out saying that you can't have duplicate field names.

We should allow users to control how VectorFields are constructed (i.e., what their component fields are, what interp method is associated with it, and what the name of the resulting vectorfield should be).