Fix import of committee report duplicates

Apparently some committee reports are divided and inserted into the Riksdagen database in parts.
 - All committee reports that are divided have a 'tempbeteckning', e.g. 'd1','d2', etc.
 - The first part has the correct 'dok_id' while the subsequent parts have appended e.g. 'd2' to the 'dok_id'
 - Subsequent parts refer to the first part through the 'relaterat_id' column
 - Selecting only document entries with empty 'relaterat_id' seems to give one and only one entry per document

Closes #45.