survivalRate(colIndex) {
        colIndex = Math.min(colIndex, this._survivalRates.length - 1);
        return colIndex < 0 ? 1 : this._survivalRates[colIndex];
    }
Determines how often the circuit evaluation survives to the given column, without being post-selected out.

Note that over-unitary gates will increase this number, so perhaps 'survival rate' isn't quite the best name.

@param {!int} colIndex
@returns {!number}