Slight C++ Restructuring

[nab170130]

Made non-void SetFunction member functions pure virtual to placate compiler errors; updated other member functions to match function signature.

Notes:

• MutualInformation, ConditionalGain have their maximize, getEffectiveGroundSet functions uncommented. If these are not meant to perform any behavior, then perhaps a NULL return type or something else should be returned here.
• Pytest passes all but 4 test cases: -- test_sparse_cpp_eval_groundset[LogDeterminant] (Main branch fails on Colab as well) -- test_sparse_py_eval_groundset[LogDeterminant] (Main branch fails on Colab as well) -- test_naive_lazy[LogDeterminant] (My branch fails uniquely on my machine) -- test_stochastic_lazierThanLazy[LogDeterminant] (Main branch fails on Colab as well)
• pip install -i https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple/ submodlib is not changed; may need to be updated after merge
• I did not fix the warnings, but I was able to install submodlib using pip install .

[vishkaush]

The changes look good. However, I just noted that the the tests/test_all.py in the repo right now had some tests commented out. So probably all tests have not yet run. Can you pull my commit cb877233f615f9d8ad1b5cd1861a30794946d243 and rerun the tests? A total of 685 tests should run and the following five (only) should fail. They are actually normal, but wrongly reported as failures right now. FAILED tests/test_all.py::TestAll::test_sparse_cpp_eval_groundset[LogDeterminant] FAILED tests/test_all.py::TestAll::test_sparse_py_eval_groundset[LogDeterminant] FAILED tests/test_all.py::TestAll::test_stochastic_lazierThanLazy[LogDeterminant] FAILED tests/test_all.py::TestAll::test_mi_dense_cpp_eval_groundset[GraphCutConditionalGain] FAILED tests/test_all.py::TestAll::test_mi_dense_py_eval_groundset[GraphCutConditionalGain] Once all tests pass (Except the above five), this will be good to merge.