Add unit tests for writing to S3 stores

[pont-us]

At present, there are only a couple of very trivial unit tests for S3 access (checking behaviour with invalid parameters). We should add more thorough tests for writing to S3 stores, probably using the moto library for mocks.

[forman]

Instead of adding new unit tests, I'd prefer we get rid of any s3-specific code. We can do this by relying entirely on the fsspec package and auto-configuration through keyword injection from config YAML. Finally we have less code and need to test.

@pont-us I'd therefore like to add the "wontfix" label and close this one in favor of #46.

[pont-us]

@pont-us I'd therefore like to add the "wontfix" label and close this one in favor of #46.

Agreed and done.