String representations of SGRID layouts

[VeckoTheGecko]

I was doing some exploring of Xarray internals (i.e., using indices), and I quickly whipped this up so that we can visualise SGRID metadata that's attached to any dataset to help with my debugging.

Example:

In [1]: from parcels._core.utils import sgrid
   ...: from parcels._datasets.structured.generic import datasets_sgrid
   ...: ds = datasets_sgrid['ds_2d_padded_low']
   ...: metadata = sgrid.parse_grid_attrs(ds.grid.attrs)
   ...: print(metadata)
<ipython-input-1-860a92e3be2e>:1: UserWarning: This is an alpha version of Parcels v4. The API is not stable and may change without deprecation warnings.
  from parcels._core.utils import sgrid
Grid2DMetadata
  X-axis:  face='face_dimension1'  node='node_dimension1'  padding=low
  Y-axis:  face='face_dimension2'  node='node_dimension2'  padding=low
  Z-axis:  face='vertical_dimensions_dim2'  node='vertical_dimensions_dim1'  padding=low
  Coordinates: lon, lat

  Staggered grid layout (symbolic 3x3 nodes):

    ↑ Y                     ↑ Z
    |                       |
    n --u-- n --u-- n       w
    |       |       |       |
    v   ·   v   ·   v       ·
    |       |       |       |
    n --u-- n --u-- n       w
    |       |       |       |
    v   ·   v   ·   v       ·
    |       |       |       |
    n --u-- n --u-- n --→ X w

  n = node  (node_dimension1, node_dimension2)
  u = x-face  (face_dimension1)
  v = y-face  (face_dimension2)
  w = z-node  (vertical_dimensions_dim1)
  · = cell centre

  Axis padding:

  face_dimension1:node_dimension1 (padding:low)
    ─────●─────●─────●─────●─────●
      1  1  2  2  3  3  4  4  5  5

  face_dimension2:node_dimension2 (padding:low)
    ─────●─────●─────●─────●─────●
      1  1  2  2  3  3  4  4  5  5

  vertical_dimensions_dim2:vertical_dimensions_dim1 (padding:low)
    ─────●─────●─────●─────●─────●
      1  1  2  2  3  3  4  4  5  5

Future work:

• Explore how to more easily surface this to users (e.g., via an xarray accessor - print(ds.sgrid.metadata))
• Integrate this into our documentation (i.e., using executed cells)

[erikvansebille]

Nice tool! Two small comments below