Multilabel model

No memory leaks in the unit-test.
Output of ```valgrind ./shogun-unit-test --gtest_filter=MultilabelModel.*``` is : 

```
==19962== HEAP SUMMARY:
==19962==     in use at exit: 0 bytes in 0 blocks
==19962==   total heap usage: 55,968 allocs, 55,968 frees, 3,577,241 bytes allocated
==19962==
==19962== All heap blocks were freed -- no leaks are possible
==19962==
==19962== For counts of detected and suppressed errors, rerun with: -v
==19962== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 1 from 1)
```