move attributes to separate dimensions

Learned from PyBOS 
```
    dataset.x.attrs["units"] = unit
    dataset.y.attrs["units"] = unit
    dataset.U.attrs["units"] = unit
    dataset.V.attrs["units"] = unit
    dataset.U.attrs["long_name"] = "Displacement along x"
    dataset.V.attrs["long_name"] = "Displacement along y"
```