JuliaExperimental: simplify conversion of matrices

[fingolfin]

... by using the functionality now built into GAP.jl

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.67%. Comparing base (3eb636d) to head (b9c7bf6).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1346      +/-   ##
==========================================
+ Coverage   78.03%   78.67%   +0.63%     
==========================================
  Files          61       61              
  Lines        4945     4924      -21     
==========================================
+ Hits         3859     3874      +15     
+ Misses       1086     1050      -36     

Files with missing lines	Coverage Δ
pkg/JuliaExperimental/gap/hnf.g	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.