Implement the `positive` argument in all relevant functions

[kdpsingh]

yardstick.event_first is now deprecated so we will (respectively) force users to supply a positive class as a character value in the argument.

[kdpsingh]

Hi @aarjun94, can we work on this issue next?

Some of the other issues are related to this issue, so once we fix this it'll be easier to fix others. Let me know if you have questions about what is needed here.