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.

[chennes]

@aiksiongkoh can you double-check this PR?

[aiksiongkoh]

@aiksiongkoh can you double-check this PR?
I believe the PR is correct. The correct version is of code is
void GEFullMat::backSubstituteIntoDU()
{
answerX = std::make_shared<FullColumn>(n);
answerX->at(n - 1) = rightHandSideB->at(m - 1) / matrixA->at(m - 1)->at(n - 1);
for (ssize_t i = (ssize_t)n - 2; i >= 0; i--) //Use ssize_t because of decrement
{
auto rowi = matrixA->at(i);
double sum = answerX->at(n - 1) * rowi->at(n - 1);
for (size_t j = i + 1; j < n - 1; j++)
{
sum += answerX->at(j) * rowi->at(j);
}
answerX->at(i) = (rightHandSideB->at(i) - sum) / rowi->at(i);
}
}