use pooch for downloading, caching, checking files

[ahms5]

as discussed in #807:

how about using pooch.retrieve to download and cach all the files, e.g. in pyfar.signals

pros:

• we dont need to think how to chach it
• we do md5 hash checks, to make sure everthing went right.
• seems like the cache would be the same for each installation of pyfar, so we just need to download it once. And not everytime again, if reinstall it.

cons:

• another dependency

[ahms5]

I think we discussed it for the Gallery, but it might also be helpful here in pyfar

[artpelling]

FYI https://github.com/fatiando/pooch/issues/473 It seems like, at least currently, there is not a lot of hope for extended support of repositories that are not currently implemented. I think pooch is still nice and pooch.retrieve is probably sufficient for most use cases.