add get_row_key to DataTable

[saska]

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

Reasoning:

I'd like to

1. Get the currently selected row with cursor_row
2. Update the table with some new data
3. move_cursor to whichever row the cursor was previously on by row_key (and fall back to index if it doesn't exist)

And this seems like an omission from the current API unless there's a reason for it not to be there?