cad/lib3mf: Add lib3mf

lib3mf is a C++ implementation of the 3D Manufacturing Format file
standard.

It provides 3MF reading and writing capabilities, as well as
conversion and validation tools for input and output data. lib3mf
runs on Windows, Linux and MacOS and offers a clean and easy-to-use
API in various programming languages to speed up the development
and keep integration costs at a minimum.

As 3MF shall become an universal 3D Printing standard, its quick
adoption is very important. This library shall lower all barriers
of adoption to any possible user, let it be software providers,
hardware providers, service providers or middleware tools.

The specification can be downloaded at http://3mf.io/specification/.