add cheops stimuli to StimulusFactory vsc[#minor]

[anilbey]

• added PosCheops & NegCheops
• Updated the notebooks
• function name apply_multiple_step_stimuli became apply_multiple_stimuli
• Protocols are added to the docs

cc @stefanoantonel

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.23%. Comparing base (ad05c99) to head (b07f7fd).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #172 +/- ## ========================================== + Coverage 91.18% 91.23% +0.05% ========================================== Files 95 95 Lines 5592 5626 +34 ========================================== + Hits 5099 5133 +34 Misses 493 493 ```

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