p_square_quantile incorrect for few observations

The P^2 algorithms assumes that 5 observations are available from the beginning.

The `p_square_quantile` class make the same assumption, and it always returns the middle height (index 2) even when there are less than 6 observations.

A special case is needed for count < 6.

A solution is to always sort the heights when count < 6. Then we can return something like this (where `cnt` is the current count)

```
result_type result(dont_care) const
{
if (cnt >= 6)
{
return this->height[2];
}
else
{
const std::size_t nearest_rank = std::ceil(cnt * p);
return this->height[nearest_rank - 1];
}
}
```