ENH: unpack example data files locally

[NickleDave]

Added example annotation files in a crowsetta.data package in #90 but because of the way importlib.resources works, when a user calls crowsetta.data.get('format-name') it returns a context manager -- this is required since packages may be installed as a compressed file, e.g. a zip, and so there's no real path on the file system to a file that's still compressed inside that .zip.

The downside of this is that it can give the impression that a user always has to use a context manager.
It also makes all of the examples in the docstrings very verbose.

Here's a different approach:

• when crowsetta.data.get is called, it checks if there is an existing place on filesystem with all of the example files unpacked there
• it uses a default argument but if a user really wants to they can change this argument; the trade-off is they will need to pass in an alternate location every time they call
• if files do not exist, and user consents, then they will be unpacked there using importlib.resources
• then / otherwise the requested sample annotation file is returned

I think I can copy some code from pooch to do this -- they have a notion of "default download location"

[NickleDave]

• [ ] add feature as described
• [ ] change all examples in docstrings
  • this would additionally require making it so examples assume that data is already unpacked locally as described ... some sort of setup code for doctest?