Move ncbi_taxon_id from biomaterial_core to donor_organism

[hannes-ucsc]

biomaterial_core.ncbi_taxon_id refers to a species.

Biologically speaking, only donor organisms can be of a species and all biomaterials must have been derived from a donor organism at some point. Therefore, all biomaterials have an implicit species: the same species as that of the donor they are derived from. Defining ncbi_taxon_id in biomaterial_core allows for anomalous metadata graphs where a biomaterial and the donor it was derived from have contradicting NCBI taxon IDs. The schema should be modeled such that it does not allow for anomalous graphs.

Moving ncbi_taxon_id from biomaterial_core to donor_organism would eliminate the possibility of these anomalous graphs. It would require that every graph contains at least one donor and that every biomaterial in that graph is directly or indirectly connected to a donor.