Complex gcd

[Jordi Gutiérrez Hermoso]

As I announced to Jaroslav privately over chat, I implemented the
complex version of the gcd. The algorithm is naïve, and reading Pari's
source, probably not very optimised. Then again, I wonder who really
wants highly optimised number theoretical functions in Octave.

I also fixed a small bug that wasn't distinguishing the output
coefficients of the extended Euclidean (or Bezout's) algorithm.

Patch attached.

HTH,
- Jordi G. H.

gcd.diff
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