Fix merged nodes

[dorbarker]

Fixes an issue that can occur when the taxonomy database is newer than the kraken2 database. Sometimes taxid nodes get merged. The merged nodes are removed from nodes.dmp and noted in merged.dmp.

So if you've generated your kraken2 results with the latest pre-built databases, but have run make_ktaxonomy.py with a freshly downloaded taxonomy, make_kreport.py will fail because it finds taxids that are not in the taxonomy.

This PR will add the obsolete taxid as a child of the updated taxid. This allows make_kreport.py to still run if the situation described above arises.