Pytzer: the Pitzer model for chemical activities and equilibria in aqueous solutions in Python

Changes in v0.5.1

• Fixed pz.convert.solvent_to_solution to take and return pK values instead of K values, and to replace any NaNs in the input with 0.
• Added pz.equilibrate.stoichiometric.get_constants() to calculate stoichiometric equilibrium constants for the carbonic acid equilibria when total dissolved inorganic carbon is zero.