function gauss(direction, taps = 5, samplerNum) {
    return new BlurExpr(direction, taps, samplerNum);
}
creates expression that performs one pass of a gaussian blur
@param direction direction to blur (keep magnitude less than or equal to 1
for best effect)
@param taps number of taps (defaults to 5)
@param samplerNum which channel to sample from (default 0)