Add sqrt and cbrt functions for QQAbElem

[aaruni96]

Adds sqrt and cbrt functions for QQAbElem. Resolves #3011 .

[thofma]

roots(a, n) returns a vector, whereas sqrt and cbrt should return a single element. Also tests would be good :)

[codecov]

Codecov Report

Merging #3096 (177ead3) into master (6d999d6) will increase coverage by 0.00%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3096   +/-   ##
=======================================
  Coverage   80.51%   80.52%           
=======================================
  Files         524      524           
  Lines       70412    70423   +11     
=======================================
+ Hits        56694    56705   +11     
  Misses      13718    13718           

Files	Coverage Δ
src/Rings/AbelianClosure.jl	94.87% <100.00%> (+0.09%)	⬆️

... and 3 files with indirect coverage changes

[thofma]

Thanks. I suggest two minor changes to the tests to make them more robust (we do not guarantee which root is returned).

May @fieker also wants to have a look?