Add a PrintObj method for general domains which know their GeneratorsOfDomain

[ssiccha]

Supersedes #3460.

This enables nicer printing of mappings between sets:

gap> MappingByFunction(Domain([1..5]), Domain([1..5]), x -> x);
MappingByFunction( Domain([ 1 .. 5 ]), Domain([ 1 .. 5 ]), function( x ) ... end )

[ChrisJefferson]

This looks reasonable -- the failing tests is because the manual needs adjusting to the new nicer output.

[fingolfin]

Looks good to me in principle, but of course the tests need to pass.

[ssiccha]

Ah, there were tests in the manuals that I missed. I'll have a look.

[ssiccha]

Updated. It should work now.

[codecov]

Codecov Report

Merging #3468 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3468      +/-   ##
==========================================
+ Coverage   85.34%   85.34%   +<.01%     
==========================================
  Files         699      699              
  Lines      346483   346487       +4     
==========================================
+ Hits       295711   295720       +9     
+ Misses      50772    50767       -5

Impacted Files	Coverage Δ
lib/relation.gd	100% <ø> (ø)	⬆️
lib/domain.gi	90.5% <100%> (+2.8%)	⬆️
src/stats.c	95.17% <0%> (-0.01%)	⬇️

[coveralls]

Coverage increased (+0.002%) to 85.169% when pulling af56913 on ssiccha:ss/add-PrintObj-method-for-general-domains into 4437e09 on gap-system:master.