Possibility of an initial multi-phase equilibrium example

[stmcgovern]

Opening an issue to keep track of discussion of the possibility/feasibility of a liquid/gas multi-phase simulation in CabanaMD. FYI @lifflander

[streeve]

Hey, I think this is a great idea - until I have some time to look more into this, here's a relevant paper
https://doi.org/10.1063/1.4720089