New mutable interfaces
Added a function to vet a MutableGraph
Added a function to vet a MutableGraph
Made Graph a MutableGraph
Refactoring to complete MutableGraph
Minor interface fixes
Added mutable directed graph
File name changes
Fixed Johnson to use MutableDirectedGraph as dummy
Updated copy functions
Fixed min span tree
Fixed name of New function in mutdir
Fixed tests for new mutable interface
Merge branch 'mutable-changes' of github.com:gonum/graph