feat: Add __str__() methods to EventDataFrame and GazeDataFrame

[josephine-funken]

Description

Added string representations to EventDataFrame and GazeDataFrame

Fixes issue # 448

Implemented changes

Insert a description of the changes implemented in the pull request.

• [X] Add EventDataFrame.__str__() method which includes frame in output
• [X] Add GazeDataFrame.__str__() method which includes experiment and frame in output
• [X] Remove the use of frame from all tutorial notebooks

Type of change

• [X] New feature (non-breaking change which adds functionality)

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] New and existing unit tests pass locally with my changes
• [X] Any dependent changes have been merged and published in downstream modules
• [X] I have checked my code and corrected any misspellings

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (38e93da) to head (befc4f6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #727 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 60 60 Lines 2813 2823 +10 Branches 714 715 +1 ========================================= + Hits 2813 2823 +10 ```

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