PHREQC engine enhancements

[rkingsbury]

Summary

Misc. enhancements to the PHREEQC implmentation

• Retain the phreeqpython solution instance in between calls, only re-instantiating when the composition changes. This helps considerably with performance.
• Refactor NativeEOS and PhreeqcEOS to eliminate redundant code via inheritance
• remove equilibrate_phreeqc()

[codecov[bot]]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (0c749b8) 78.57% compared to head (5c4ea1b) 78.50%.

Files	Patch %	Lines
src/pyEQL/engines.py	85.71%	6 Missing and 3 partials :warning:
src/pyEQL/functions.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #81 +/- ## ========================================== - Coverage 78.57% 78.50% -0.07% ========================================== Files 10 10 Lines 1456 1442 -14 Branches 248 249 +1 ========================================== - Hits 1144 1132 -12 - Misses 273 274 +1 + Partials 39 36 -3 ```

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