Fix von Mises-Fisher sampler

[devmotion]

Fixes #1423.

As already discussed in #1423, in the special case mu = [1, 0, ..., 0] no rotation of the "initial" sample is needed. I think skipping the rotation completely is preferable over a numerical workaround such as #1918.

I used the opportunity to also add comments and references to the existing code of the von Mises-Fisher sampler.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.02%. Comparing base (3de6038) to head (ca68082).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1930      +/-   ##
==========================================
+ Coverage   85.99%   86.02%   +0.02%     
==========================================
  Files         144      144              
  Lines        8685     8702      +17     
==========================================
+ Hits         7469     7486      +17     
  Misses       1216     1216              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.