Fixed a bug when removing excluded datasets

[toncho11]

Fixed a bug when removing excluded datasets in benchmark.py.

Solves #654

The only commit is [Fixed a bug when removing excluded datasets in benchmark.py]

[toncho11]

It accepts both which does not much my code.

[toncho11]

Example of using the PR.
exclude_datasets uses class names as strings in the case of this PR .

results = benchmark(
    pipelines=pipeline_folder,
    evaluations=["WithinSession"],
    paradigms=["P300"],
    exclude_datasets=["EPFLP300","FakeVirtualRealityDataset","Huebner2017","Huebner2018","Lee2019_ERP","Sosulski2019"],
    results="./results/",
    overwrite=False,
    plot=True,
    n_jobs=4,
    output="./benchmark/",