Cross-assembling weights for difference reference databases causes issues

[BenKaehler]

In some instances it is useful to assemble weights using amplicons from one set of primers, then use them to train a classifier for a amplicons from a different set of primers.

If we do that at the moment, it causes issues downstream because the sets of taxa are slightly different for the trimmed reference data sets.

So, it would be good to have a utility that takes a training reference database and a target reference database and makes sure that the training reference database cannot generate taxa that are not in the target reference database.

The cross-reference database so assembled could then be used to train the uniform classifier that is responsible for assigning taxa prior to assembling weights.