Add `Chain.set_data` signature to set many/all draws at once

[michaelosthege]

This is needed for copying draws/stats from other structures, or other chains:

https://github.com/pymc-devs/pymc/blob/2a324bc04d65637f427858c519f2eb9c247ebc0b/pymc/backends/mcbackend.py#L188-L195