Add pymbar citations to duecredit

[IAlibay]

We should add those when we call multistate_analysis or just load up from results.

[richardjgowers]

pymbar is currently always used with the Protocol? So it could just be stuffed alongside the existing citations we'll add for everything

[mikemhenry]

I think if there isn't a way to use the public API for a protocol without hitting pymbar, its fine to stuff on the protocol level, but if there is a way to skip using it, then we will need that granularity.

In the case of pymbar, since it is a package under my maintenance umbrella, we can upstream duecredit support (at least for pymbar4).