ENH/FIX: Convergence improvements for dilute case

[richardotis]

Fixes gh-271.

• Sets MIN_SITE_FRACTION = 1e-16 (previously 8e-13)
• Changes the minimum allowed composition condition to a constant, 1e-10, instead of 1000*MIN_SITE_FRACTION (previously 8e-10)
• Fixes test_degree_of_ordering to have more significant digits of precision
• Adds allowed_mass_residual inside of the minimizer, which automatically adjusts the mass residual tolerance for dilute calculations

[codecov]

Codecov Report

Merging #332 (c33b469) into develop (db5086c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #332   +/-   ##
========================================
  Coverage    87.34%   87.35%           
========================================
  Files           46       46           
  Lines         4521     4524    +3     
========================================
+ Hits          3949     3952    +3     
  Misses         572      572           

Impacted Files	Coverage Δ
pycalphad/core/constants.py	100.00% <100.00%> (ø)
pycalphad/core/equilibrium.py	94.32% <100.00%> (+0.12%)	⬆️
pycalphad/tests/test_equilibrium.py	97.14% <100.00%> (ø)
pycalphad/tests/test_model.py	100.00% <100.00%> (ø)

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