Add DataTree.match_names to match node names

[Armavica]

I came up with another function name that the one proposed in the issue, but happy to change that if subset_nodes sounds better.

• Closes Subset variable names from nodes xarray-contrib/datatree#232
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[welcome]

Thank you for opening this pull request! It may take us a few days to respond here, so thank you for being patient.
If you have questions, some answers may be found in our contributing guidelines.