1.12.65: allow aq cat to display template library templates

[jouvin]

The template library being installed in the plenary template area, it is not easy to see its contents. It is possible to go to the sources on GitHub or maintain a private copy but it would be handy if aq cat could display a template from the template library with something like:

aq cat --template_library --version 18.6.0 --subset standard -- template hardware/cpu/intel/xeon_e5_2670v3.pan

--template should be the name known by the user (relative to the loadpath). --subset starndard|core|os|..., --core, --os... would allow to specify a template library subset. Ideally, it should be possible not to specify it and have every subset be tried until one match is found but it would not work for OS, cloud, grid where there are multiple versions behind so it is probably easier to require it in the first implementation.

It would be good also to have a --list that will give the template list available in the namespace specified by --template. This would be useful to know the existing hardware templates in the template library, for example.