Insoluble salts in galvanic elements

[andreasdh]

I think it would be better if the standard salts in the galvanic elements were nitrates instead of sulphates to avoid ending up with insoluble salts when drawing the elements.

Also, the method "diagram" does not longer work (although "draw" works), but it is still used in the documentation.

[harirakul]

Thanks for the suggestion. I have updated the documentation with the correct info about the "draw" method.