BUG: repeated calls to equilibrate cause pH changes

[rkingsbury]

repeatedly calling equilibrate with the native engine (and presumably the PHREEQC one also) causes the pH to change, even when there have been no changes to the composition or conditions. This should not occur.

>>> s1 = Solution({'Na+': '2 mol/L','Cl-': '2 mol/L'},temperature='20 degC',volume='500 mL')
>>> for i in range(10):
...     print(s1.pH)
...     s1.equilibrate()
...
7.0
6.954575819099865
6.885161416260718
6.818765085545055
6.7555886088307755
6.695795267325627
6.639519561022746
6.586873954955933
6.5379537003727615
6.492840366549978