[pathLabel] Add a label to a path

pathLabel.asy

void pathLabel(
    picture pic=currentpicture,
    Label l,
    path g,
    real position=0.5,
    align align=NoAlign,
    bool sloped=false,
    pen p=currentpen,
    filltype filltype=NoFill
) {
    Label l2 = Label(l, align, p, filltype, position=Relative(position));
    if (sloped) {
        pair direction = dir(g, reltime(g, position));
        real angle = degrees(atan2(direction.y, direction.x));
        l2 = rotate(angle) * l2;
    }
    label(pic, l2, g);
}

// Examples
// pathlabel("BeginPoint", p, position=0);
// pathlabel("Relative(0.25)", p, position=0.25);
// pathlabel("MidPoint", p, position=0.5, align=Relative(W), sloped=true);
// pathlabel("EndPoint", p, align=Relative(E), position=1.0);