Ensure (ux)fieldset has both UV and UVW for datasets with U, V, and W

[fluidnumerics-joe]

Summary

This branch ensures that FieldSet objects created from datasets containing U, V, and W velocity components have both UV and UVW vector fields available.

Changes

Bug Fix: Missing UV VectorField for 3D velocity datasets

Previously, when a dataset contained all three velocity components (U, V, W), only the UVW VectorField was created. This meant users could not access a 2D UV VectorField for horizontal-only advection without manually creating one.

Now, both UV and UVW VectorFields are automatically created when all three components are present, providing flexibility for both 2D and 3D advection use cases.