Add FileDownloader class with file:// protocol

[gyger]

Add FileDownloader class with file protocol to allows repository to be e.g. on a network share

Path needs to start with file://.

Relevant issues/PRs:
See #429

[santisoler]

Hi @gyger. Thanks for opening this PR. Sorry for the delayed reply, I spent last week at a conference.

I'll take a look at this soon.

[santisoler]

Hi @gyger! This new class looks good! I think it would be a nice addition to Pooch.

A few things that we need to add before we can merge this PR:

• Unit tests. We would need to add some tests that checks that the downloader works as expected. We could use pytest's tmp_path to create temporary locations from where files will be downloaded from.
• Add the new class to the api/index.rst, under the Downloaders section.
• Import the new class in pooch/__init__.py, so it becomes available at the top module level.

It would also be nice to include an example in the documentation on use cases for this new downloader.

Let me know if you can and are willing to address these points. Feel free to ask for help if you need it.