Support detached crates

[simleo]

Fixes #34.

See ResearchObject/ro-crate#189, but note that the current spec already allows a root data entity id other than ./ (see #34).

Changes the find_root_entity_id method to support the general case where the metadata descriptor @id is an absolute URIs whose last path segment is ro-crate-metadata.json[ld]. Despite being able to handle a more general situation, the new implementation is actually much simpler and more efficient in the "standard" case of an attached RO-Crate, where the metadata descriptor @id is ro-crate-metadata.json[ld]: in this case, the search ends with an O(1) lookup on the existing entity map. If this fails, it looks for candidate URIs whose last path segment is ro-crate-metadata.json[ld]. If there are no candidates, it raises a KeyError; if there's only one candidate, it's selected as the metadata entity; if there's more than one, it tries to filter out those that don't satisfy all constraints (e.g., type is not CreativeWork). If there's still more than one candidate, it issues a warning and picks one using a heuristic.

[simleo]

Discussed at yesterday's RO-Crate meeting: agreed to issue a warning before trying to make a choice with a heuristic when there is more than one good candidate.