Draft: Do not build element dicts but element rows for CSV export

• Make it actually work 🙂 things are not in the right order probably because this cool looking thing

    for column in metadata_columns:
            element_row.append(
                next(
                    (item.value for item in element_md if item.name == column),
                    None,
                )
            )

  does not do what I want it to; maybe this did:

  if with_metadata and metadata_columns:
        element_md = element_metadata(item.id, load_parents=with_parent_metadata)
        # For each column name append metadata value or an empty string in the correct column order
        for column in sorted(set(metadata_columns)):
            mds = [md.value for md in element_md if md.name == column]
            column_count = len([col for col in metadata_columns if col == column])
            assert len(mds) <= column_count, "Metadata and column count mismatch"
            i = 1
            for md in mds:
                element_row.append(md.value)
                i += 1
            while i < column_count:
                element_row.append("")

• tests
• docs