Faster Jonker-Volgenant Assignment Algorithm

This updated version performs the JV algorithm for a standard assignment problem for only valid entries in the cost matrix.

The modification removes rows and columns of the matrix where all values are inf, which is useful for applications where it is necessary to mask off certain assignments, such as in the k-best assignment algorithm (Murty algorithm).

The improvement in the runtime is dependent upon how many rows/cols are masked, but can be as 7-20x for many cases.

Yi Cao agreed to let me post this update, and the majority of the code is his.