JOSS submission

• Joss paper review
• Documentation update
• Most of the methods implement solve interface using XXXProblem and XXXResult structures.

v0.2.6

Changelog

• Make initial solution optional in Transportation problems
• Add tests for the shortest path problem
• Add a $100 \times 100$ test problem for assignment
• Replace Array{Type, 1} and Array{Type, 2} with Vector{Type} and Matrix{Type}, respectively
• Implement isbalanced and balance for Assignment Problem
• Solve unbalanced assignment problems