Add new GeneralBindingModel API that accepts/returns concentrations instead of log concentrations

[jchodera]

It is inconvenient to only feed the current GeneralBindingModel log concentrations, since sometimes some concentrations are identically zero and we would have to carefully remove those from the reaction equations.

Instead, it would be useful to encapsulate this behavior automatically in a new API for GeneralBindingModel, and maybe move the old binding model to a different method.

cc: #127