adding base_schema_path to the converter

[djarecka]

I'm wondering if this could be accepted as an enhancement to the schemasheets. If yes, I would add to the documentation and do some extra checks.

The idea is that I have some classes that are written in yaml and I would like to add them to the new schema without transferring them first to the schema form. Right now I'm adding this by myself after running schemasheets, but I thought that I might try to slightly generalize it and suggest the addition.

[cmungall]

Looks good! The title says [wip] but I assume you forgot to remove this, as it looks ready (usually marking as Draft is fine to entail WIP)

[djarecka]

I was thinking about adding documentation for this, but I can also add a separate PR with more changes to the documentation