Rename MCState `n_discard` to `n_discard_per_chain`

Created by: PhilipVinc

I should have done this as part of #706 but I did not think about it.

n_discard Is going to raise deprecation warnings for now, and will be removed at a later date.

This is a simple renaming and does not change the default value (which is still 10% of total samples).