Provide an alternative backend for the inventory

[MatteoVoges]

I think reclass is a bit deprecated and is really hard to extend. It's poorly documented, it's quite annoying to have it as a submodule and we don't even use all the features.
My solution to this problem would be to move to modern tools (python packages) that work with ruamel.yaml because it is maintained. I don't have any specific tools in mind, but some suggestions are:

• yamlpath
• figgy
• omegaconf
• hiyapyco