fixnan(src) {
        if (this.na(src[0])) {
            for (var i = 1; i < src.length; i++) {
                if (!this.na(src[i])) {
                    src[0] = src[i]
                    break
                }
            }
        }
        return src
    }
For a given series replaces NaN values with
previous nearest non-NaN value
@param {TS} src - Input time-series
@return {TS}