Simplify JuliaToGAP methods

[fingolfin]

Avoid using CallJuliaFunctionWithKeywordArguments

[codecov]

Codecov Report

Merging #938 (075dc47) into master (b216185) will increase coverage by 0.09%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #938      +/-   ##
==========================================
+ Coverage   75.64%   75.74%   +0.09%     
==========================================
  Files          51       51              
  Lines        4171     4151      -20     
==========================================
- Hits         3155     3144      -11     
+ Misses       1016     1007       -9     

Files Changed	Coverage
pkg/JuliaInterface/gap/convert.gi	75.00%

[ThomasBreuer]

Nice.
(First I had thought that CallJuliaFunctionWithKeywordArguments could disappear, but no, just the better syntax for julia_to_gap allows us to avoid it here.)