Add vector_fields parameter to FieldSet creation methods

[VeckoTheGecko]

Description

This PR adds a vector_fields parameter which allows users to control vector fields that are created.

Looking at the following tests you can see how they're used

def test_fieldset_structured_vectorfield_default():
    ds = datasets_structured["ds_2d_left"][["U_A_grid", "V_A_grid", "grid"]].rename({"U_A_grid": "U", "V_A_grid": "V"})

    fset = FieldSet.from_sgrid_conventions(ds, mesh="flat")

    assert "U" in fset.fields
    assert "V" in fset.fields
    assert "UV" in fset.fields


def test_fieldset_structured_vectorfield_custom():
    ds = datasets_structured["ds_2d_left"][["U_A_grid", "V_A_grid", "grid"]].rename({"U_A_grid": "U", "V_A_grid": "V"})
    ds = ds.rename({"U": "U_wind", "V": "V_wind"})

    fset = FieldSet.from_sgrid_conventions(ds, mesh="flat", vector_fields={"UV_wind": ("U_wind", "V_wind")})

    assert "U_wind" in fset.fields
    assert "V_wind" in fset.fields
    assert "UV_wind" in fset.fields


def test_fieldset_structured_vectorfield_none():
    ds = datasets_structured["ds_2d_left"][["U_A_grid", "V_A_grid", "grid"]].rename({"U_A_grid": "U", "V_A_grid": "V"})

    fset = FieldSet.from_sgrid_conventions(ds, mesh="flat", vector_fields=None)

    assert "U" in fset.fields
    assert "V" in fset.fields
    assert "UV" not in fset.fields

Now users can:

• specify the names of vectorfields and their components
• explicitly set that no vector fields are created (by doing vector_fields = {} or vector_fields = None)

If vector_fields is not provided it uses the default behaviour of constructing UV and UVW if the component fields are present.

These changes have also been added to the unstructured grid side as well.

Checklist

• Closes Decide on API for defining VectorFields #2695
• Tests added
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
  • Describe how you used it (e.g., by pasting your prompt): only for updating the docstring

[VeckoTheGecko]

NOTSET here is an example of a "sentinel value".

They're handy in the case of wanting to distinguish between None and "the default behaviour"

in our case here we want vector_fields=None to mean no vector fields, and the default behaviour to autodiscover based on if there are U and V fields.

[erikvansebille]

Thanks for this quick PR! See below some comments

[erikvansebille]

Not for this PR, but would there be a benefit in also using this NOTSET when postponing setting the particle.time?

Parcels/src/parcels/_core/particleset.py

Lines 89 to 91 in 32a7f2a

	if time is None or len(time) == 0:
	# do not set a time yet (because sign_dt not known)
	time = np.array(np.nan)

[VeckoTheGecko]

I'm not quite sure what you mean by this. Do you mean to use NOTSET instead of None for the time parameter in the init of the particleset?

[VeckoTheGecko]

All feedback has been addressed - ready for re-review