Location of signature

[laurentsimon]

We currently store the signature under the model's root directory as model.sig. If we ever want to store more information (e.g., a file with a list of directory / file hashes like sha256sum output), the model's directory tree may become bloated. So maybe we should think of creating a dedicated folder security or signatures under the root directory. This way we can store all information there.

Thoughts?

[laurentsimon]

Closing this in favor of #111