FIX: Gibbs phase rule compliance

[richardotis]

Fixes gh-468.

In some cases the outer loop of the global minimizer (eqsolver.pyx) will add phases that exceed the number allowed by the Gibbs phase rule. Infeasible solutions passed to the minimizer aren't, themselves, a problem, and can ultimately help us find minimum-energy solutions. However, the minimizer must ensure that the final solution returned is compliant.

Prior to this change, the minimizer could become 'stuck' with a solution that violates the Gibbs phase rule and is also metastable with respect to the sampled grid of points. The minimizer would nonetheless (falsely) converge and return, e.g., a four-phase stable equilibrium in an isothermal-isobaric binary system. It would then be unable to recover, either returning the non-physical solution or raising a difficult-to-interpret error.

The fix is two-fold: First, change_phases is usually only called when the convergence criteria are otherwise satisfied so, in this change, when a Gibbs phase rule violation of the candidate solution is detected, the minimum tolerated phase amount is increased by several orders of magnitude (currently from 1e-9 to 1e-4). This will force smaller-amount composition sets to be automatically removed, resetting the convergence, and continuing the optimization. For the case reported in gh-468, this fixes the issue, and should generalize to similar scenarios.

Second, as a failsafe, Gibbs phase rule compliance is explicitly checked at the end of run_loop to prevent the minimizer from reporting converged=True for any violating (non-physical) case we've failed to catch with the revised logic.

Thanks @tobiasspt for reporting.

[codecov]

Codecov Report

Merging #470 (97e917b) into develop (e345631) will increase coverage by 0.02%.
The diff coverage is 92.30%.

@@             Coverage Diff             @@
##           develop     #470      +/-   ##
===========================================
+ Coverage    90.29%   90.32%   +0.02%     
===========================================
  Files           50       50              
  Lines         7768     7781      +13     
===========================================
+ Hits          7014     7028      +14     
+ Misses         754      753       -1     

Impacted Files	Coverage Δ
pycalphad/core/minimizer.pyx	84.59% <75.00%> (+0.25%)	⬆️
pycalphad/tests/test_equilibrium.py	98.47% <100.00%> (+0.02%)	⬆️

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[bocklund]

@Zhyrek this may fix some issues you've been running in to