Remove externally-hosted diskdf corrections from the tests and docs

[jobovy]

GitHub finally seems to have stopped hosting the pre-calculated diskdf corrections that were available here:

https://github.com/downloads/jobovy/galpy/galpy-dfcorrections.tar.gz

so because these aren't crucial anyway, we remove any mention of them in the docs. They were also used in the tests (but, it turned out, weren't really necessary), so we remove them there as well. This did require re-saving the pre-calculated diskdf corrections that are included in the package as binary files, because apparently they weren't and this was not platform-independent.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (4ccd97a) 99.83% compared to head (8539390) 99.83%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #564   +/-   ##
=======================================
  Coverage   99.83%   99.83%           
=======================================
  Files         196      196           
  Lines       28598    28598           
=======================================
  Hits        28551    28551           
  Misses         47       47           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.