Add UxDataArray.data_location property

[rajeeja]

Overview

Adds a public UxDataArray.data_location property that reports where a data
variable is stored on the grid, addressing the long-open issue #547 (opened in
October 2023).

What it does

data_location infers the storage location from the grid dimension present on
the data variable and returns one of:

• "face_centered" — data contains the n_face dimension
• "node_centered" — data contains the n_node dimension
• "edge_centered" — data contains the n_edge dimension
• None — data is not mapped to the grid

This builds on the existing internal _face_centered() / _node_centered() /
_edge_centered() helpers and complements the existing data_mapping property.

Scope note

The original issue proposed a richer enumeration including face_average,
edge_orthogonal, edge_parallel, cgll, and dgll (with polynomial order
for the spectral-element cases). Those locations cannot be inferred from a data
variable's dimensions alone and are not currently representable in UXarray, so
this PR implements the three locations that are reliably determinable today and
documents the rest as not-yet-distinguished. Happy to extend the enum if/when
the underlying metadata becomes available.

Changes

• Add UxDataArray.data_location property (uxarray/core/dataarray.py)
• Add it (and the existing data_mapping) to the API reference (docs/api.rst)
• Add unit tests covering all three locations plus the non-grid / extra-dim
  cases (test/core/test_dataarray.py)

Closes #547