when calculate centroid, fixed number

[maodouchen]

TDigest.prototype._addweight = function(nearest, x, n) {
// add weight at location x to nearest centroid. adding x to
// nearest will not shift its relative position in the tree and
// require reinsertion.
//
if (x !== nearest.mean) {
nearest.mean += n * (x - nearest.mean) / (nearest.n + n);
nearest.mean = nearest.mean.toFixed(number) // add this line?
}
nearest.cumn += n;
nearest.mean_cumn += n / 2;
nearest.n += n;
this.n += n;
};