Improve Data Exploration notebook (#368)

* suggestions

* [skip ci] Fix for suggestions

- Updated the path for the happiness CSV file in the data exploration notebook to reflect the correct directory structure.
- Added hints for visualizing year values in matplotlib plots.
- Improved documentation for data cleaning functions, clarifying the merging process and filling strategies.
- Enhanced test cases to assert the return types and structure of DataFrames in the testing suite for better validation.
- Adjusted layout settings in the helper module to ensure consistent legend sizing in visualizations.

* [skip ci] Ai improvements

* writing better tests

* Changes to bonus exercise explaination

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fixing precommit requirement

* Apply suggestions from code review

Co-authored-by: Aliaksandr Yakutovich <aliaksandr.yakutovich@gmail.com>

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Co-authored-by: Despina Adamopoulou <despoina.adamopoulou@empa.ch>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Aliaksandr Yakutovich <aliaksandr.yakutovich@gmail.com>

Author: 5 people