fix: construct sparse matrix using topological sort order
Merge branch 'master' into fix/sparse_matrix

* master:
  Added tests.
  Fixed a copy/paste error.
  make DataGrabber and SelectFiles use human order
  Update indexsidebar.html
  Fixed Frontiers citation
  Update .travis.yml
Merge pull request #747 from satra/fix/sparse_matrix

fix: construct sparse matrix using topological sort order