Minor cleanup
Minor cleanup

* Clarify code in `diff_from_species`
* Remove unused return value from `validate_required_fields`

recklessromeo/otm-core,clever-crow-consulting/otm-core,maurizi/otm-core,RickMohr/otm-core,clever-crow-consulting/otm-core,clever-crow-consulting/otm-core,clever-crow-consulting/otm-core,recklessromeo/otm-core,recklessromeo/otm-core,RickMohr/otm-core,maurizi/otm-core,RickMohr/otm-core,RickMohr/otm-core,maurizi/otm-core,recklessromeo/otm-core,maurizi/otm-core