Add associated_studies onto Biosample and make part_of not required for Biosample

[brynnz22]

From the fix to model the parent/child relationships for DataGeneration, we created a slot called associated_studies so the part_of slot could be freed up to be used for the parent/child modeling. We need to stay consistent and move the studies in the part_of slot on Biosample into the associated_studies slot, which means we need to add the associated_studies slot on the Biosample class.

Remove part_of from Biosample.