Allow GAP.Obj(x,true) for recursive conversion

[fingolfin]

This makes it much more convenient to use from GAP: Julia.GAP.Obj(x,true)

TODO: document it somewhere, add a test

[codecov]

Codecov Report

Merging #910 (124d5d1) into master (38af454) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #910      +/-   ##
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- Coverage   76.47%   76.43%   -0.04%     
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  Files          51       51              
  Lines        4166     4168       +2     
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  Hits         3186     3186              
- Misses        980      982       +2     

Impacted Files	Coverage Δ
src/constructors.jl	96.20% <0.00%> (-2.50%)	⬇️

[ThomasBreuer]

Concerning documentation, the obvious places are the docstrings for GAP.Obj and GapObj, which mention currently the keyword argument recursive.

However, this affects only the use from Julia. Since the remark mentions explicitly that the new syntax is intended for the use from GAP, the point is to mention it in the JuliaInterface manual. Currently we are advertising the GAP function (constructor) JuliaToGAP. If I understand the idea of this pull request right then it would be better to advertise Julia.GAP.Obj, for example because it covers also extensions from Oscar.jl, which JuliaToGAP does not know.

[ThomasBreuer]

@fingolfin Shall we merge this pull request, and then I create a new pull request for improving the documentation?

[fingolfin]

OK!