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[bot]]

Codecov Report

Merging #25 (08f44ff) into master (d6f94d8) will not change coverage. The diff coverage is 50.00%.

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

Impacted Files	Coverage Δ
phreeqpython/solution.py	83.87% <50.00%> (ø)

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