PyMBAR 4 convergence issues

[IAlibay]

We're seeing convergence issues on very innocuous transformations when using PyMBAR 4, e.g. lig_ejm_31 to lig_ejm_50 from the tyk2 set.

We'll need to dig into it and work out why this is happening.

For now we'll temporarily pin to <4 so we can get folks started.