Fix hierarchy of plant structure absent classes #132
Description
The hierarchy of the plant structure absent classes that existed in the 2019 release files was lost in the transition to using the ODK.
Example of absent hierarchy from the 2019 release:
In issue #116 (PR #120), we managed to fix the hierarchy of the "present" terms by changing the parent class for all traits from their direct parent to 'plant structure presence'. However, this change did not fix the hierarchy of the "absent" terms.
Note: Ramona already confirmed that the removal of the GCAs did not affect the hierarchy.