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* ``Block.row`` and ``Block.col`` are now set by their containing
``BlockGrid`` at the time the ``Block`` is returned to the user.
This means that while iterating over a subgrid the first ``Block``
will have coordinates (0, 0).
* Getting a single row from a ``BlockGrid`` will return a new ``BlockGrid``
* Raise a TypeError when indexing a ``BlockGrid`` with a length one tuple
* Added a ``Block.colors`` property