Update select_unique_ngs() to get unique NGS reports at the dx level as opposed to the pt level

[jalavery]

select_unique_ngs() is currently based on selecting a unique NGS per patient (among the diagnoses subset on cohort_ngs), which is almost always going to be the structure of the diagnosis dataset returned from create_analytic_cohort(), so this is perfect for now. It is technically possible for someone to pull multiple cancer diagnoses per patient, and those cancer diagnoses would have different NGS reports and we wouldn't want to restrict to 1 NGS report across all dx, so I am going to open an issue to remind us that a future enhancement could be updating select_unique_ngs() to be unique at the diagnosis rather than at the patient level.