Compare image signature of embeddings and current image

[constantinpape]

We may want to compare the image signature for the current image and the embeddings when calling _validate_embeddings in order to make sure the user hasn't switched images without updating embeddings.

@lufre1 has implemented a version of this in #498, but I decided to deactivate this now because there were a few unclear cases and the overhead of computing the data signature every time segment or so is called would be quite large.