add stabilizer for G-sets w.r.t. derived actions

[ThomasBreuer]

The stabilizer of a Set of points in the G-set is the setwise stabilizer. The stabilizer of a Vector or Tuple of points in the G-set is the pointwise stabilizer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.48%. Comparing base (4ab6f4d) to head (942e0af).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4265      +/-   ##
==========================================
- Coverage   84.60%   84.48%   -0.12%     
==========================================
  Files         640      641       +1     
  Lines       85117    85445     +328     
==========================================
+ Hits        72011    72186     +175     
- Misses      13106    13259     +153     

Files with missing lines	Coverage Δ
src/Groups/gsets.jl	92.15% <100.00%> (+0.51%)	⬆️

... and 17 files with indirect coverage changes

[mjrodgers]

I think this is good as a first implementation, and all of the remarks of @lgoettgens seem to be addressed.

I think as a next step maybe we can implement these without computing the entire group action on sets/tuples; but I think we should merge this now and then I will open a separate issue for performance improvements.