document Julia.GAP.Obj(x,true), add tests

[ThomasBreuer]

This is the promised follow-up of #910.

[codecov]

Codecov Report

Merging #925 (8e68d03) into master (47e264d) will increase coverage by 0.04%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #925      +/-   ##
==========================================
+ Coverage   75.53%   75.58%   +0.04%     
==========================================
  Files          51       51              
  Lines        4165     4165              
==========================================
+ Hits         3146     3148       +2     
+ Misses       1019     1017       -2     

Files Changed	Coverage Δ
pkg/JuliaInterface/gap/convert.gd	100.00% <ø> (ø)
src/types.jl	80.00% <ø> (ø)

... and 1 file with indirect coverage changes

[ThomasBreuer]

The test OscarCI / matching: [Oscar#master] - ubuntu-latest, julia ~1.6.0-0 (pull_request) fails:

du Val Tester: Error During Test at /home/runner/work/GAP.jl/GAP.jl/oscar-dev/Oscar/test/AlgebraicGeometry/Schemes/duValSing.jl:12
  Test threw exception
  Expression: is_du_val_singularity(X, J3) == false
  Ideal does not describe a single K-point

This is almost the same error as in the failures of oscar-system/Oscar.jl/pull/2410 and oscar-system/Oscar.jl/pull/2528.

[fingolfin]

This is oscar-system/Oscar.jl#2531 -- it is a bug in Singular that needs to be fixed there, which should be in the works

[ThomasBreuer]

Thanks @fingolfin. Then I regard this pull request as ready for review.
(Is it a reasonable idea to document Julia.GAP.Obj in the JuliaInterface manual as if it were a GAP function?)

[fingolfin]

(Is it a reasonable idea to document Julia.GAP.Obj in the JuliaInterface manual as if it were a GAP function?)

IMO it is1