feat: introduce measure_samples(), MeasureLibrary and null_ratio

[dkrako]

Description

This introduces a first measure using the GazeDataFrame.measure_samples() method. The measures calculation is grouped by trial columns.

Implemented changes

• [x] GazeDataFrame.measure_samples()
• [x] MeasureLibrary
• [x] null_ratio measure

Type of change

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

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [x] tested null_ratio() with None and nan
• [x] tested if MeasureLibrary has registered null_ratio
• [x] tested GazeDataFrame.get_measure() with and without groupby

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 (348acee) to head (64eb644).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #673 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 57 60 +3 Lines 2705 2757 +52 Branches 698 709 +11 ========================================= + Hits 2705 2757 +52 ```

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