feat: susie_finemapper_ss_gathered()

[Daniel-Considine]

✨ Context

🛠 What does this PR implement

Allows SusieFineMapperStep to run directly on a study_locus_row that has already had the gwas summary statistics for the locus annotated with StudyLocus.annotate_locus_statistics()

🙈 Missing

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[addramir]

Looks good to me. Works im my environment too.