Use another dataset for tests and notebooks

[eonu]

Tests currently rely on small simple datasets that aren't really representative of even a simple real-world dataset.

Notebooks rely on TorchFSDD which is a good dataset, but requires installation of torch in order to run tests. This can be an issue in terms of compatibility on some devices, and we can get away with a simpler dataset.

Perhaps the solution to both of these problems is #141.

Note: Make sure to fix the tests that were skipped in #201. Also examples on RTD need to be updated.

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• Update notebooks ([add:lib] Add sequentia.datasets module #214)
• Update examples on RTD ([add:lib] Add sequentia.datasets module #214)
• Update tests ([patch:test] Update HMM tests to use datasets module #217)

[eonu]

Fixed in #217.