Allow query cluster assignment without taxonomy

[Robaina]

So far, labelplacements.py only works for MARdb sequences with valid MMP identifiers (so a taxonomic path can be retrieved from the GTDB database). However, this method prevents cluster assignment to query sequences when reference sequences don't come from MARdb.

This issue will modify:

https://github.com/Robaina/TRAITS/blob/7ca208a0bb593dea9ebec5c20a99877e7d60ad3f/code/phyloplacement/placement.py#L371-L389

to allow taxonomy-free cluster assignment.

To this end, we can create a fake taxopath only containing the cluster name (perhaps additional fields if required, it doesn't matter). Gappa examine assign should then assign clusters to placed queries as if it had a valid taxopath

[Robaina]

Pull Request #34 solved this issue, clusters can now be assigned to queries independent of taxonomy