Improve handling of GPU IDs

Improve handling of GPU IDs

Shifted responsibility for handling parsing of mdrun -gpu_id to early
in the runner, rather than as part of the assignment process.

Moved utility string handling + tests to taskassignment module, since
they only supported this process. Updated string handling in gmx
tune_pme to use more std::string and use the new
functionality. makeGpuIds will be used to replace the code in
assign_rank_gpu_ids in a subsequent patch.

Change-Id: I8d39cc69d0f96ac395858ed7cbe9f2947081b384