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