NativeEOS: ensure equilibrate does not change solvent mass

[rkingsbury]

This PR fixes an issue in which repeated calls to equilibrate when using NativeEOS or PHREEQCEOS would
change the mass of the Solution slightly. This was attributed to the fact that pyEQL and PHREEQC use slightly
different molecular weights for water.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 82.19%. Comparing base (934db1f) to head (f275c06).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/pyEQL/engines.py	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
- Coverage   82.28%   82.19%   -0.09%     
==========================================
  Files          10       10              
  Lines        1456     1460       +4     
  Branches      293      294       +1     
==========================================
+ Hits         1198     1200       +2     
- Misses        220      221       +1     
- Partials       38       39       +1     

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