Vendorize skimage

[nbren12]

Scikit-image is a difficult dependency to manage (especially since it makes strong assumptions about pooch). Even an import of scikit-image mutates the users ~/.cache folder, which can lead to hard to debug permissions errors and other issues. We work around this via careful dependency pinning, but I still frequently run into this problem when e.g. updating versions. We only use two small functions from scikit-image, so it's easy to vendorize them in this repo.

We can reimplement this using xarray.coarsen at another time (see #1048).

Added public API:

• vcm.testing.checksum_dataarray
• vcm.testing.checksum_dataarray_mapping

Significant internal changes:

• rewired the coarening tests to use regression checksums instead.

Requirement changes:

• removed scikit-image
• I would've liked to remove pooch too, but metpy uses it too, albeit only in one module that we probably don't import.

[spencerkclark]

It's great this was so simple to vendor over. I just have a minor comment about the modification to the test of _xarray_block_reduce_dataarray.