shower nozzle support

There is little chance this thing can be reused by anyone. It is a very specific repair. A piece of plastic that broke some times ago I had to replace without buying the whole equipment.

I nevertheless publish it because it is (from my point of view) an interesting usage of the great Openscad library called [Relativity](https://www.thingiverse.com/thing:349943).
It is the first time I use this library. Without it the scad code was ugly. It is now much simpler.

I had to adapt relativity in order for rod() primitive to accept two radius/diameter parameters like cylinder() does. As stated in the documentation, it is possible to make a cone with two hulled() rod(). But I was not able to subtract such a hulled() part from another.