Move mean calculation out of loop

[IDisposable]

In the variance calculation, it recalculates the mean in each pass of the loop, leading to and On² execution time. Moved the invariant calculation outside the loop to make this On instead.

[simogeo]

this.mean() is calculated once and stored for further usage, see code :

/** return mean value */
    this.mean = function() {

        if (this._nodata())
            return;

        if (this.stat_mean  == null) {

            this.stat_mean = parseFloat(this.sum() / this.pop());

        }

        return this.stat_mean;
    };

Anyway, your PR is welcome since it will to avoid tests. Thanks !

[IDisposable]

I missed the memorization, sorry.