feat: decision tree for FM in fine_mapper

[Daniel-Considine]

✨ Context

Currently we aren't filtering out loci with too few loci prior to fine-mapping

🛠 What does this PR implement

After the join between summary statistics and LD index, if there are less than 100 variants an error is logged and the locus is not fine-mapped.

🙈 Missing

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[addramir]

I like the logic and I agree we should keep the filtering in function, not init function as I suggested before.

1. These changes should be merged to new function with LD interface
2. We need to add the same case but for extremely large number of variants, I suggest 15K