Fix mass conservation for H+ and OH- in Solution.__add__ following #270

[rkingsbury]

#270 introduced a change to Solution.__add__ that inadvertently broke mass conservation of H+ and OH- species when adding solutions. This PR fixes that issue. In brief, __add__ now relies on conservation of mass of H+ and OH- in the components attribute to determine the new pH of the mixed solution.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.63%. Comparing base (e6dde9d) to head (bf9cee1).
⚠️ Report is 99 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #274      +/-   ##
==========================================
+ Coverage   81.90%   83.63%   +1.72%     
==========================================
  Files           9        9              
  Lines        1503     1460      -43     
  Branches      261      251      -10     
==========================================
- Hits         1231     1221      -10     
+ Misses        226      210      -16     
+ Partials       46       29      -17     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.