Define a struct cone type and include the radius of the base r and the
height h as members. Write two functions ComputeArea() computing the area and
ComputeVolume() computing the volume of a given cone. Write another
function NewCone() that inputs a cone and a radius r. The function should return
a new cone whose volume is the same as the input cone but whose rabius of
base is r. Define PI as a constant.

Example:
My cone is r=3 h=4.

ComputeVolume() must return 36.
ComputeArea() must return 72.
NewCone() must return the new cone with r=1 and h=36 for my input r
being 1.