Improve m0/m1 computation

[tzaeschke]

m0/m1 computation in the quadtree iterators could be improved by replacing

for (int d = 0; d < center.length; d++) {
    m0 <<= 1;
    m1 <<= 1;
    if (max[d] >= center[d]) {
        m1 |= 1;
        if (min[d] >= center[d]) {
            m0 |= 1;
        }
    }
}

with

for (int d = 0; d < center.length; d++) {
    m0 <<= 1;
    m1 <<= 1;
    //Extract signum bit
    m1 |= (Double.doubleToRawLongBits(center[d] - max[d]) >>> 63) & 1;
    m0 |= (Double.doubleToRawLongBits(center[d] - min[d]) >>> 63) & 1;
}