FIX: potential sorting bug in lower_convex_hull

[bocklund]

Fixes a sorting bug introduced into lower_convex_hull by #329. We used to rely on indep_conds for sorting the state variables, but now we use the state_variables variable directly and indep_conds is dead code.

This PR sorts the state_variables now.

[codecov]

Codecov Report

Merging #339 (f81a257) into develop (4e399bb) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #339   +/-   ##
========================================
  Coverage    84.25%   84.25%           
========================================
  Files           45       45           
  Lines         4529     4529           
========================================
  Hits          3816     3816           
  Misses         713      713           

Impacted Files	Coverage Δ
pycalphad/core/lower_convex_hull.py	97.72% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4e399bb...f81a257. Read the comment docs.