feat!: enforce list of strings for GazeDataFrame.trial_columns attribute

[dkrako]

Description

Although GazeDataFrame accepts a single string for passing trial_columns, enforcing a list of strings as the resulting attribute GazeDataFrame.trial_columns simplifies internal usage in #655

As a side effect, this could also get rid of the depraction warning in #667

Implemented changes

• [x] create list of string in case trial_columns is of type string

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• [x] add test for str
• [x] add test for list of single str
• [x] add test for list of two strings

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:

Comparison is base (8f17a97) 100.00% compared to head (194d867) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #669 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 54 54 Lines 2366 2366 Branches 590 590 ========================================= Hits 2366 2366 ```

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