*Specification:*

We want to realize a static public method that takes as parameters two positive
integers *x* and *y* and computes and returns their greatest common divisor
gcd(*x*, *y*).

The greatest common divisor of two integers *x* and *y* is the greatest integer
that divides both *x* and *y* without rest.

*Example:*

gcd(12, 8) | = | 4 |

gcd(12, 6) | = | 6 |

gcd(12, 7) | = | 1 |