summary.optmatch() using MCFSolutions info

[benthestatistician]

Info to pull from the MCFSolutions material:

• sum (or mean) of matched distances
• bound on discretization regret

[benthestatistician]

As discussed in this comment, there's a performance argument for postponing the evaluation of the dual objective until user asks for a summary of the object. But some tradeoffs are noted a bit lower in the same comment thread; whether this change gets made may hinge on resolution of #85.