Merge pull request #1 from basilegithub/cleaning_code

Merged.

Fixed Gaussian elimination crash

Fixed an index problem in the gaussian_elimination.c file that caused the function to crash after reducing the relations set.

Optimizations and error correction

Worked to optimize the sieving step mostly.
One error was made in the code, in the second loop of the case 1 in the sieving function : the formula to compute the second starting point actually returned a negative number. Since root is unsigned long, this meant that its value was actually very big, thus the program was never doing the sieving with the second root. Correcting this yielded a 2x speedup.
Optimizations in the sieving function (mostly avoiding mpz functions when possible) yielded a 2x speedup.
Overall, I see a ~ 4x speedup.

Fixed problematic function call

A function call in reduce_matrix.c caused the program to crash during runtime on linux. This is modified to some other function call that works on linux and windows.

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