Use return value of CloseMutableBasis to replace not IsContainedInSpan

[stertooy]

As suggested in gap-system/gap#6288.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.82%. Comparing base (618dfd4) to head (ef43307).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
- Coverage   55.82%   55.82%   -0.01%     
==========================================
  Files          87       87              
  Lines       12678    12676       -2     
==========================================
- Hits         7078     7076       -2     
  Misses       5600     5600              

Files with missing lines	Coverage Δ
gap/matrep/matrep.gi	84.50% <100.00%> (-0.09%)	⬇️

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