Add GazeDataFrame.to_csv()

[dkrako]

Description of the problem

Currently we do not have saving functionality on the level of a single gaze file.
We only have GazeDataFrame.save() which saves files of the complete dataset.

Description of a solution

The implementation is very straight forward and can be build up on the code from the internal function
save_preprocessed()

Minimum acceptance criteria

• implement GazeDataFrame.to_csv()

Sample Code

Calling the method with an existing GazeDataFrame should be as easy as gaze.to_csv().

Context

Please provide any additional relevant information.