Add support for multiprotic acids and bases

[pm-blanco]

We have users interested on setting up simulations with multiprotic acids and bases. While this should be possible to be done within pyMBE, we need to do a few adjustments:

• Generalize the number of states of chemical species in the dataframe, for now we hard code it to 4.
• Add unit tests to check that the setup of multiprotic acids and bases works properly
• Sample script showcasing how to setup a multiprotic acid or base in pyMBE
• (optionally) functional test to benchmark that the implentation works in the future.

We should also think if there is any generalization we should do when combining the GRxMC method with multiprotic acids or bases. @gnshb has volunteered to develop this feature.