is_left and is_right for SubgroupTransversal

[fingolfin]

Also use is_left/is_right for those and cosets

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.49%. Comparing base (194d82b) to head (16cc394).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Groups/cosets.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4298      +/-   ##
==========================================
- Coverage   84.49%   84.49%   -0.01%     
==========================================
  Files         641      641              
  Lines       85528    85525       -3     
==========================================
- Hits        72269    72264       -5     
- Misses      13259    13261       +2     

Files with missing lines	Coverage Δ
src/Groups/cosets.jl	96.02% <90.00%> (+0.04%)	⬆️

... and 3 files with indirect coverage changes

[ThomasBreuer]

Looks good.