Camera Lucida (in OpenSCAD)

This is a very basic camera lucida, an optical drawing tool used by artists to assist when sketching a picture.

Background

I was intrigued when I first saw the [neoLucida](https://neolucida.com/) and thought that it would be something I'd like to try - until I saw the price... a bit much when I wasn't sure that I'd like using it.

I had done some looking around and found a couple of interesting designs on Printables: the [first one](https://www.printables.com/model/526409-lucida-3d-portable-camera-lucida) by Chris Borge looked splendid, until I realised that the base and lid were too long for either of my printers; the [second one](https://www.printables.com/model/695381-basic-camera-lucida), also by Chris Borge, was far simpler to print but I didn't have a suitable old lamp base. Because of these problems I blatantly pinched a few ideas from both and added my own ideas!

The head unit is almost identical to the simple camera lucida, and the use of the telescopic arm was ta