Fix off-by-one in GEFullMat::backSubstituteIntoDU()

[fuzzybear3]

Body:

Summary

If you look at this PR #12 It had already recommended this change but seemed to have been overlooked.

Some info from Claude:

• Fix out-of-bounds vector access in GEFullMat::backSubstituteIntoDU() where answerX->at(n) and rowi->at(n) accessed index n in
  vectors of size n (valid range: 0 to n-1)

Details

GEFullMat::backSubstituteIntoDU() initializes the back-substitution correctly on line 23 using n - 1:
answerX->at(n - 1) = rightHandSideB->at(m - 1) / matrixA->at(m - 1)->at(n - 1);

But the first iteration of the loop used n instead of n - 1:
double sum = answerX->at(n) * rowi->at(n); // out of bounds

This is the same class of bug fixed in GESpMatFullPv::backSubstituteIntoDU() and causes a vector::_M_range_check exception at runtime.

GEFullMat is the base class for GEFullMatParPv and GEFullMatFullPv, neither of which override backSubstituteIntoDU(), so this affects
both full-pivot and partial-pivot dense matrix solves.