ProblemData interface should inherit from Map<String, ProblemDataItem>

[jfilak]

Otherwise it doesn't make sense to define

abstract void add(String key, String value);

since the parent interface defines this method.

[mozeq]

After some discussion with jfilak we came to a conclusion that it's ok to implement Map<String, String>, but the class should follow the Map naming convention so additional methods are easy to find.