Allow specifying external resources in placegroups

[TonioF]

Originally brought up in #926 . Placegroups at the moment expect a path parameter, which must point to a file on the local filesystem.
In some cases it is desirable though to load in a file from an external resource, e.g., from a s3 bucket. It would be good if this option existed, possibly by defining a datastore that serves vector data.