function yFromLstar(lstar) {
    return 100.0 * labInvf((lstar + 16.0) / 116.0);
}
Converts an L* value to a Y value.

L* in L*a*b* and Y in XYZ measure the same quantity, luminance.

L* measures perceptual luminance, a linear scale. Y in XYZ
measures relative luminance, a logarithmic scale.

@param lstar L* in L*a*b*
@return Y in XYZ