ENH: Model: Change some uses of subs to symbol_replace for performance

[richardotis]

This makes a significant performance difference for multicomponent models with magnetism.

[codecov]

Codecov Report

Merging #340 (d862ecf) into develop (6219d4d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #340   +/-   ##
========================================
  Coverage    84.25%   84.25%           
========================================
  Files           45       45           
  Lines         4529     4529           
========================================
  Hits          3816     3816           
  Misses         713      713           

Impacted Files	Coverage Δ
pycalphad/model.py	91.82% <100.00%> (ø)

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