function gaussian(x, a = 0, b = 1) {
    return new GaussianExpr(expr_1.wrapInValue(x), expr_1.wrapInValue(a), expr_1.wrapInValue(b));
}
gaussian function that defaults to normal distribution
@param x x position in the curve
@param a horizontal position of peak (defaults to 0 for normal distribution)
@param b horizontal stretch of the curve (defaults to 1 for normal distribution)