Implement minimum cost multicommodity flow algorithm

Using Lemon Graph Library (http://lemon.cs.elte.hu/trac/lemon) for minimum cost flow subroutine. Minimum cost flow algorithm from Google OR tools also works but slower.

Todo:

1. Control the error bound with numerical error analysis
2. Leverage the old states of network simplex for faster min-cost flow subroutine.

References:

1. Karger, David, and Serge Plotkin. "Adding multiple cost constraints to combinatorial optimization problems, with applications to multicommodity flows." STOC. Vol. 95. 1995.
2. Leighton, Tom, et al. "Fast approximation algorithms for multicommodity flow problems." Journal of Computer and System Sciences 50.2 (1995): 228-243.
3. Goldberg, Andrew V., et al. "An implementation of a combinatorial approximation algorithm for minimum-cost multicommodity flow." International conference on integer programming and combinatorial optimization. Springer, Berlin, Heidelberg, 1998.