Interactions with site archives and forkserver

[tennlee]

Note for myself to follow up on - when creating the data loading module with multiprocessing_context=forkserver, the forked instances aren't importing the site archives when loading pipelines. This could be a bug, or maybe we need to add an optional and/or automagical option into the pipelines to make them aware of the need for pre-imports.

The site archives "register" site-specific module classes into the pyearthtools.data.archive namespace, and this needs to occur before deserialising the rest of the pipelines, where they draw on those archives.