consider formalisation of any-subyt-implementation

[marc-portier]

This templating approach has been ported to php using https://twig.symfony.com/

It would be nice if we can have some formal description and validation/testkit for what the 'subyt' way of working actually entails
(using a new repo, that should (mostly) be python-indepent?)

goal would be to formally describe / standardise the

• filters
• the template patterns
• maybe even some cli arguments

and package that up into formats (inputs, templates, results) that can be used in different implementations to test compliance