Enable injecting Python generated Stimuli [vcs: #minor]

[anilbey]

Adds a layer to create stimuli in Python and combine them with + operator. It does not use NEURON and it is not dependent on the Cell module. The current trace of each generated stimuli can be retrieved consistently.

Changes

• Add StimulusFactory enabling Python generated Stimulus creation
• Stimulus creation through StimulusFactory is decoupled from the Cell object
• Cell.add_step and Cell.add_ramp use StimulusFactory
• Move stimuli module to stimulus/circuit_stimulus_definitions
• [DOCS] Add list of stimuli page that describe, generate and plot the stimuli
• Add jupyter notebook displaying how to inject the StimulusFactory Stimulus into Cell

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.61654% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 89.23%. Comparing base (d7a5240) to head (e39fd4d). Report is 11 commits behind head on main.

Files	Patch %	Lines
tests/test_stimulus/test_factory.py	94.89%	5 Missing :warning:
bluecellulab/stimulus/factory.py	97.41%	3 Missing :warning:
bluecellulab/cell/injector.py	95.65%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #142 +/- ## ========================================== + Coverage 85.30% 89.23% +3.92% ========================================== Files 80 87 +7 Lines 5404 5387 -17 ========================================== + Hits 4610 4807 +197 + Misses 794 580 -214 ```

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[stefanoantonel]

Looks good to me! If you can just put an example of how to reproduce at least the patterns that we see on the OpenBrainPlatform that would be helpful