rare events detection in impution pipeline

[rfeng2023]

We put the rare events detection in phenotype imputation pipeline for now, as we need to do imputation first and then normalization.In principle this should be put into normalization part, but we would lost the rare events index once got imputation. To do in future:

1. add those rare events detection in each normalization pipeline
2. keep rare events index after imputation and do that filter in normalization pipeline.