add is_valid_lei()

[cjyetman]

I built this to make sure the example LEI I was using was valid. Not sure if it has much use beyond that, but maybe? Should it be exported, or only internal? @jacobvjk @jdhoffa

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[jdhoffa]

I built this to make sure the example LEI I was using was valid. Not sure if it has much use beyond that, but maybe? Should it be exported, or only internal? @jacobvjk @jdhoffa

I wonder if this makes more sense to have in r2dii.data?
Seems useful there also (in the generation of mock data), and could still be leveraged in pacta.loanbook since it depends on r2dii.data