Miscellaneous small fixes

[artivis]

Miscellaneous small fixes:

• group == operator constness
• tangent constexpr variables
• tl::optional missing return value
• examples: init random generator and reduce noise
• disable macos-11 in CI since the image is not ready yet (see (X) This check failed - sometimes happens on macOS runner actions/runner-images#841)
• delete Travis config file since we're not using it anymore

[codecov]

Codecov Report

Merging #200 (74c57c6) into devel (48ba49d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            devel     #200   +/-   ##
=======================================
  Coverage   98.37%   98.37%           
=======================================
  Files          49       49           
  Lines        1475     1475           
=======================================
  Hits         1451     1451           
  Misses         24       24           

[joansola]

See some stupid remarks in the comments. Then I-ll approve.

[artivis]

@joansola Can I get an approval to merge this PR as soon as CI is done?

[joansola]

All agreed