admit gap_to_julia conversion to Vector{GAP.Obj}

[ThomasBreuer]

This addresses issue #740, but I am still not sure what would be a good solution.

[ThomasBreuer]

The point is that Vector{GapObj}(L) for a GAP list L of Julia objects starts with gap_to_julia.
In the recursion, we want to convert the list entries to GapObj, which means that we have to switch to julia_to_gap.
A "natural place" for expressing this generically would be in src/gap_to_julia, where the conversion of GAP.GapObj and GAP.Obj for recursion situations is handled, but I do not see how to formulate in general what one wants.
(Perhaps the idea to specify a "direction" of conversion as in gap_to_julia and julia_to_gap is not a good idea at all.)

[codecov]

Codecov Report

Merging #849 (cfd5f3a) into master (ad3e631) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #849      +/-   ##
==========================================
- Coverage   81.46%   81.40%   -0.07%     
==========================================
  Files          48       48              
  Lines        3664     3667       +3     
==========================================
  Hits         2985     2985              
- Misses        679      682       +3     

Impacted Files	Coverage Δ
src/gap_to_julia.jl	91.97% <100.00%> (+0.11%)	⬆️
src/julia_to_gap.jl	96.93% <100.00%> (+5.18%)	⬆️
src/setup.jl	75.86% <0.00%> (-3.45%)	⬇️
src/GAP.jl	76.57% <0.00%> (-2.71%)	⬇️
src/help.jl	32.55% <0.00%> (-2.33%)	⬇️

[fingolfin]

I'd prefer to finish PR #777 which should also deal with this.

I'll try to prioritize that next week. We'll see.