Allow block_split and BlockReduce to take a shape

[leouieda]

block_split and BlockReduce/BlockMean (which relied on it) only
allowed defining the block size by spacing. Now they an take a shape
as well. Useful when the size of the block is less meaningful than the
number of blocks (like in spatial cross-validation splitting).

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and verde/__init__.py.
• Write detailed docstrings for all functions/classes/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.