Make sc_id optional. Closes #16.
Make sampling_units_file optional. Closes #17.
Make recruitment_type_id optional. Closes #18.
Make core uri optional. #19.
Add checks when sampling_unit_file is not present.#17
Add minimally valid hash and add/edit tests for #16, #17, #18, #19.
ROC-related changes are new features. #16, #17, #18, #19.
Merge pull request #20 from NUBIC/non_required_fields

Make sc_id, recruitment_type_id, sampling_units_file and core uri optional.