Add counter-ion swaps for maintaining charge neutrality

[bas-rustenburg]

Greg has added functionality to saltswap that should allow us to plug saltswappers into our Simulation style classes. This should allow the code to maintain charge neutrality.

• [ ] I should design a CounterIonProtonDrive that can interact with a salinator/swapper from saltswap code.
• [ ] We should extend functionality of ConstantPHSimulation and ConstantPHCalibration as needed to facilitate the new class.
• [ ] Add a salt reporter?

As John points out below, in the future we will also want to support compatibility with a full osmostat simulation.

[jchodera]

We have distinct functionalities we want to add:

• (1) the ability to swap a single water/ion to maintain charge neutrality during a protonation state change---we need this for the paper
• (2) the ability to alternate with salt insertion/deletion using the osmostat---we want to punt on this to a later paper

[bas-rustenburg]

Yes, thanks for pointing that out. This should cover only the maintaining of charge neutrality for now.

[bas-rustenburg]

Updated.

[bas-rustenburg]

Implemented this feature in #113.