Feature Request: Allow 'Not applicable' for tumour_grading_system and tumour_grade

[Buwujiu]

Detailed Description

As tumour grading is not applicable to certain cancers, for example blood cancers:

• T-cell Lymphoma (EUPTCL-DE)
• Multiple Myeloma (MM-KR)
  Programs would like to submit Not applicable for tumour_grading_system and tumour_grade.
  Because tumour_grade is depend on tumour_grading_system, when tumour_grading_system = Not applicable, tumour_grade shall be Not applicable as well.

After we implement this, we need to modify clinical validation rules to allow tumour_grade = Not applicable, otherwise users will see clinical validation errors, because tumour_grade is a required field.

Note that this is NOT a clinical exception, as it's a valid scientific case.

The clinical changes will be done here: icgc-argo/argo-clinical#805

Exit Criteria

when users submit the following combo to clinical system

tumour_grading_system	tumour_grade
Not applicable	Not applicable

there should be no dictionary error, this should be a valid case.

Possible Implementation

• Add a new enum value Not applicable to the permissible list of tumour_grading_system.
• Add a new enum value Not applicable to the permissible list of tumour_grade.
• Modify the tumour_grade 's validation scrip to allow Not applicable value when tumour_grading_system = Not applicable.