Add variable to data model representing study group/participant group

[alyssadai]

Examples of groups within a neurodevelopmental study could be "Typically Developing", "Sibling", "Patient".

Options for the variable/class name:

• nb:StudyGroup
• nb:ParticipantGroup
• nb:SubjectCohort -> maybe overlaps with our understanding of a cross-dataset subject cohort?
• nb:StudyCohort -> same as above
• ...

Related issues:

• Explain how to handle healthy controls documentation#225
• Revisit how queries for 'healthy controls' should be handled relative to the diagnosis param api#43
• Expose SubjectGroup as a column in participant-level results TSV query-tool#72

Questions:

• Do we need to consider links between study groups and the actual study? e.g., control for "schizophrenia" vs. control for "PD"