Add method to check component name duplication

[mmurooka]

If composite (e.g., variables, constraints, costs) contain components with the same name, it seems that an unexpected problem will arise (is this understanding correct?)
This Pull Request adds a method to check the name duplication to the Problem class. Doing the same thing in user code is difficult because variables_ is private and there is no access method.

(This is a utility that I use locally. It may be worth sharing in the origin repository, so I'll make a pull request.)

[Levi-Armstrong]

@awinkler What do you think about updating the Composite to store a std::set<std::string> of the names and when adding a component it inserts the name into the set and check the results to see if it was successfully inserted and if not throw an exception?