Clean up parameter file discovery and handling

[sobolevnrm]

Understand the issues underlying #65

[speleo3]

Maybe related?

DeprecationWarning: Use of .. or absolute path in a resource path is not allowed and will raise exceptions in a future release.
    return get_provider(package_or_requirement).get_resource_filename(

I get this when running python -m pytest tests with Python 3.8

[sobolevnrm]

Yes, the new .. error popped up recently. Not sure if it's related to what we saw initially.