Include Grid File Format as Attribute

[rytam2]

Closes #933

Overview

Added attribute that describes grid file format when loading grid with ux.grid.
self._ds.assign_attrs({"source_grid_spec": self.source_grid_spec})

PR Checklist

General

• An issue is linked created and linked
• Add appropriate labels
• Filled out Overview and Expected Usage (if applicable) sections

Testing

• Adequate tests are created if there is new functionality
• Tests cover all possible logical paths in your function
• Tests are not too basic (such as simply calling a function and nothing else)

Examples

• Any new notebook examples added to docs/examples/ folder
• Clear the output of all cells before committing
• New notebook files added to docs/examples.rst toctree
• New notebook files added to new entry in docs/gallery.yml with appropriate thumbnail photo in docs/_static/thumbnails/

[philipc2]

Nice work!