Added optional unit parameter to kinetics

[uliw]

It seems that the kinetic method always assumes that the rates returned by the rate_function are in mmol.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.50%. Comparing base (d6f94d8) to head (08f44ff).
⚠️ Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
phreeqpython/solution.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #25   +/-   ##
=======================================
  Coverage   75.50%   75.50%           
=======================================
  Files           7        7           
  Lines         698      698           
=======================================
  Hits          527      527           
  Misses        171      171           

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