Update stimulus factory API [vcs: #minor]

[anilbey]

• removes duration from the inject_sequence module
• add Flat, Empty, Slope Stimulus classes used as building blocks in defining more complicated shapes. E.g. Step = Empty + Flat + Empty Ramp = Empty + Slope + Empty
• allows Ramp to be constructed by both threshold and amplitude based
• replace start, end, post_wait with pre_delay, duration, post_delay for interpretability because to understand the duration one had to subtract start from the end in the previous implementation. Now a step is represented with 5ms delay followed by a 10ms flat and then 5ms more delay. No need to do mathematics to understand the shape.

[stefanoantonel]

I tested and they seem to work fine!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.39130% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 91.22%. Comparing base (6287fe7) to head (a297d9c). Report is 1 commits behind head on main.

Files	Patch %	Lines
bluecellulab/stimulus/factory.py	94.64%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #164 +/- ## ========================================== - Coverage 91.23% 91.22% -0.01% ========================================== Files 93 93 Lines 5497 5530 +33 ========================================== + Hits 5015 5045 +30 - Misses 482 485 +3 ```

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[anilbey]

The 3 missing lines are covered, patch coverage is %100