Implement grid-informed ``UxDataArray.isel()`` and ``UxDataset.isel()``

[philipc2]

Proposed new feature or change:

When indexing a grid dimension (i.e. n_face, n_node, n_edge), indexes the grid connectivity and returns a subset of the unstructured grid (with a new Grid object only containing those elements)

Functionality for non-grid dimensions (i.e. time) remains unchanged