Wrong series returned from _curve

[chen0040]

There is some bug in https://github.com/EthicalML/xai/blob/master/xai/__init__.py#L962

it was written as

r1s = r2s = []

but should be instead

r1s, r2s = [], []

The impact is that if the user would like to us r1s and r2s returned to construct the the curve (e.g. for storing the data for later analysis), they would find that r1s and r2s are referring to the same instance which stores all the curve data that should have been separately stored in r1s and r2s

[axsaucedo]

Interesting - thank you for flagging, would you be interested on opening a PR @chen0040 ?