Fix sample files by moving them to a new GitHub repro.

[CarlKCarlK]

This fixes the sample files (issue #48) by pointing to a new GitHub repro instead of the old AWS source.

• The repro is at https://github.com/fastlmm/bgen-sample-files. You may want to clone it into your "limix" project instead.
• This does not use Pooch. Instead, it uses the existing pooch-like code. Thus, avoid a dependency on Pooch.
• However, I think some other dependency already depends on Pooch, so let me know if you'd rather use Pooch.
• The GitHub actions fail, but not because of this change -- the original code fails, too.
• (I accidently committed this to main for a few minutes. But I reverted and everything should is now fine.)

-- Carl