script to copy previous values pheno-list.json

[majorseitan]

given two pheno-list.json files, prev and current,
will copy the specified files. the fields copied default
to num_cases, num_controls, num_gw_significant.
The new fields are given the suffix _prev.

copy_pheno_list_values.py --fields field1,field2 <source> <destination>

features

1. backup previous file
2. check columns are not overwritten.

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