v0.8.0

v0.8.0 deprecates node sum as the unique key (or hash index) to differentiate columns in the column generation module as different columns may share the same node sum. The new implementation features a side-by-side comparison on column itself with a slight improvement in both running time and convergency gap. Details can be found at here.

1. Deprecate node sum in _backtrace_shortest_path_tree() and _update_column_gradient_cost_and_flow() in colgen.py;
2. Deprecate node sum in load_columns() in utils.py and adopt the same implementation from the updated _backtrace_shortest_path_tree();
3. Rename _assignment() to _generate_column_pool() and _assignment_core() to _generate() to be consistent with their true functionalities;
4. Optimize read_links() and output_agent_paths() in utils.py;
5. Cosmetic changes in path_engine.cpp and DTALite source files with the critical change to include fstream in utils.h;
6. Add a section How to Cite;
7. Fix typos over multiple files.