Add a helper to load a GEM hardware layout tree

Summary

Once !36 (merged) will be merged, one should be able to load any configuration tree from a YAML file. GEM specific objects also do exist in gem/core/layout-tree/nodes.h.

However, since nodes must be registered into the loader instance, the usage in different locations is not straightforward. A helper should be provided in order to easily load a GEM hardware layout tree.

What is the expected correct behavior?

A GEM hardware layout tree should easily be loadable in a few lines of code.

One could also add, let's be crazy, a YAML file describing the coffin setup hardware layout as part of the documentation in the examples.