#25 Function for conical frustum

[Stanli]

I wrote a function that makes a cone frustum to be added to trunk/asymptote/base/solids.asy

// Return a right circular conical frustum of height h in the direction of axis.
// The lower base of radius r1 is centered at C, the upper base has radius r2.
revolution cone_frustum(triple C, real r1, real r2, real h, triple axis=Z)
{
    return revolution(C, (r1,0,0)--(r2,0,h), axis);
}

I'm sorry I didn't submit a pull request or something like that, but I had a hard time finding out how to do it in here.

[Stanli]

I'm sorry, I must be too tired; I gave you the wrong thing. It should be this way:

// Return a right circular conical frustum of height h in the direction of axis.
// The lower base of radius r1 is centered at C, the upper base has radius r2.
revolution cone_frustum(triple C, real r1, real r2, real h, triple axis=Z)
{
    return revolution(C,(C.x+r1,C.y,C.z)--(C.x+r2,C.y,C.z+h),axis);
}

Because the previous code din't account for xyz shifts with respect to C.