added new test for rokko::sort_eigenpairs for two majors (col, row) and
two orders (ascending, descending).
(the present version of sort_eigenpairs fails the test)
fixed sort_eigenpairs (still O(N^2))
reduced complexity of sort_eigenpairs to O(N log N).

implemented by std::sort and std::pair.
Merge pull request #78 from t-sakashita/sort_eigenpairs