equilibrate: modify total conc check for missing elements

[rkingsbury]

Summary

This PR slightly modifies the logic used to detect whether PHREEQC has discarded an element. The previous logic compared the original total element concentration with the one returned by PHREEQC, and if the two differed by more than a low tolerance, pyEQL would assume that the element had been dropped and add back in the entire original concentration, as discovered by @vineetbansal in #329 .

Although the bug has already been fixed in #351 , this PR goes a step further to check only whether the "new" total concentration (i.e., that returned by PHREEQC) is greater than zero. If it is, that indicates that PHREEQC is "aware" of the element in question and has retained it.

We retain an error message that will be thrown if the original and new total concentrations differ by more than 5%. This really should never happen, but had this error message been present in previous versions of equilibrate, it would have caught #329 and brought it to the user's attention.

[rkingsbury]

(Note to self, the USGS link https://www.usgs.gov/software/phreeqc-version-3 is timing out during the docs linkcheck. I think that's temporary and on their server side so I'm ignoring)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.53%. Comparing base (806b876) to head (12fa836).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #352      +/-   ##
==========================================
+ Coverage   85.36%   85.53%   +0.17%     
==========================================
  Files          14       14              
  Lines        1858     1860       +2     
  Branches      322      323       +1     
==========================================
+ Hits         1586     1591       +5     
+ Misses        226      224       -2     
+ Partials       46       45       -1     

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