Function count_divisors

* Get number of divisors of a given number * * If \f$x = a \times b\f$, then both \f$a\f$ and \f$b\f$ are divisors of * \f$x\f$. Since multiplication is commutative, we only need to search till a * maximum of \f$a=b = a^2\f$ i.e., till \f$\sqrt{x}\f$. At every integer till * then, there are eaxctly 2 divisors and at \f$a=b\f$, there is only one * divisor. */