update single-cell notebook with trajectories from a given cell

[kilianFatras]

What does this PR do?

This PR adds illustration examples to the single-cell notebooks where we show the trajectory difference between SF2M and SBCFM from the same cell. SB-CFM only produces one trajectory (by its deterministic nature) contrary to SF2M which produces several outputs as it models a stochastic process.

It is a novel feature.

Before submitting

• [x] Did you make sure title is self-explanatory and the description concisely explains the PR?
• [x] Did you make sure your PR does only one thing, instead of bundling different changes together?
• [x] Did you list all the breaking changes introduced by this pull request?
• [x] Did you test your PR locally with pytest command?
• [x] Did you run pre-commit hooks with pre-commit run -a command?

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Make sure you had fun coding 🙃

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Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (dbeeb75) 34.23% compared to head (64aed61) 34.23%. Report is 2 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #64 +/- ## ======================================= Coverage 34.23% 34.23% ======================================= Files 55 55 Lines 6268 6268 ======================================= Hits 2146 2146 Misses 4122 4122 ``` | [Files](https://app.codecov.io/gh/atong01/conditional-flow-matching/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alexander+Tong) | Coverage Δ | | |---|---|---| | [runner/src/datamodules/distribution\_datamodule.py](https://app.codecov.io/gh/atong01/conditional-flow-matching/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alexander+Tong#diff-cnVubmVyL3NyYy9kYXRhbW9kdWxlcy9kaXN0cmlidXRpb25fZGF0YW1vZHVsZS5weQ==) | `44.82% <100.00%> (ø)` | | | [runner/src/eval.py](https://app.codecov.io/gh/atong01/conditional-flow-matching/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alexander+Tong#diff-cnVubmVyL3NyYy9ldmFsLnB5) | `81.08% <ø> (ø)` | | | [runner/src/train.py](https://app.codecov.io/gh/atong01/conditional-flow-matching/pull/64?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alexander+Tong#diff-cnVubmVyL3NyYy90cmFpbi5weQ==) | `92.59% <ø> (ø)` | |

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