FIX: calculate: Phase index offset for associate models

[richardotis]

Fixes gh-367.

[codecov]

Codecov Report

Merging #368 (130b2f1) into develop (4461299) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #368      +/-   ##
===========================================
+ Coverage    89.91%   89.93%   +0.01%     
===========================================
  Files           44       44              
  Lines         4362     4370       +8     
===========================================
+ Hits          3922     3930       +8     
  Misses         440      440              

Impacted Files	Coverage Δ
pycalphad/core/calculate.py	92.67% <100.00%> (+0.03%)	⬆️
pycalphad/tests/test_equilibrium.py	97.70% <100.00%> (+0.04%)	⬆️

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