Move ProblemIO into core Planner API

After the benchmark has been split into its own Maven module, I need to import both -core and -benchmark into my projects when I'm using ProblemIO interface.

Since I'm using it not only inside a benchmark but also for general reading/writing of solutions (in order to be able to share the code between my planner and my benchmarks), I would appreciate if this useful interface could be part of the core API.

(Because one day I would like to distribute my planner app without the unnecessary benchmark dependencies, while still keeping the classes implementing the ProblemIO interface.)