add isomorphic_subgroups

[ThomasBreuer]

and add missing is_isomorphic method for FinGenAbGroup

[codecov]

Codecov Report

Attention: Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.40%. Comparing base (6b027f0) to head (2158b4c).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Groups/homomorphisms.jl	86.36%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4401   +/-   ##
=======================================
  Coverage   84.40%   84.40%           
=======================================
  Files         656      656           
  Lines       87246    87270   +24     
=======================================
+ Hits        73640    73661   +21     
- Misses      13606    13609    +3     

Files with missing lines	Coverage Δ
src/GAP/wrappers.jl	95.14% <100.00%> (+0.01%)	⬆️
src/Groups/homomorphisms.jl	91.73% <86.36%> (-0.22%)	⬇️

... and 1 file with indirect coverage changes

[mjrodgers]

This looks good! I'm assuming these two checks that fail are just flukes and will pass on another run.

[joschmitt]

The book test nightly failure is #4402. I restarted the julia 1.6 job.