Check if OCEL XML is valid when parsing

Currently, the `import_ocel_xml` function _always_ returns `OCEL`. 

Ideally, it should return a `Result` type (like the XES parser), and at least check if the parsed OCEL is _somewhat_ valid (e.g., contains at least one object/event types or something; Also check with https://www.ocel-standard.org/ to see what _actually_ is required in OCEL files).