Additional persisters

[ottonemo]

I have written an S3 persister (FileIO and FileLike) based on s3fs and thought about making a PR (if you are interested) but I assume that you probably don't want a direct dependency to packages such as s3fs and boto3. Two questions:

• are you interested in a PR at all
• if so, how to integrate the S3 persister without cluttering the dependencies

[BenjaminBossan]

I believe it would make sense as completely optional dependency.

[dnouri]

We're already using setuptools extras for our optional dependencies to julia, rpy2, and others. I like the idea of adding a S3 persister to Palladium. We can even put it into persistence.py, as long as the case of not being able to import these dependencies is handled gracefully.