Add __str__() methods to GazeDataFrame and EventDataFrame

[dkrako]

Description of the problem

Currently we always need to use .frame to print the dataframes in interpreters and notebooks. It would be better if both classes would have a nice string representation.

Description of a solution

Basically forwarding pl.DataFrame.__str__ should do the trick, but GazeDataFrame should also include the experiment definition.

Minimum acceptance criteria

• [ ] GazeDataFrame.__str__() method which includes experiment and frame in output
• [ ] EventDataFrame.__str__() method which includes frame in output
• [ ] remove the usage of frame from all tutorial notebooks if it is used just for displaying the dataframe