fix: operator precedence when building group_filter_expression

[saeub]

Description

If there are multiple trial columns, the group_filter_expression in GazeDataFrame.detect() is built incorrectly, leading to polars.exceptions.InvalidOperationError: bitand operation not supported for dtype str. This is because the & operator has precedence over ==.

Implemented changes

• [x] Put parentheses around the == expression

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change is or requires a documentation update

How Has This Been Tested?

• [x] tests/unit/gaze/detect_test.py::test_gaze_detect[idt_constant_position_single_fixation_per_trial_multiple_trial_columns]

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 (fa4e4db) to head (5d9d45b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #734 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 60 60 Lines 2763 2763 Branches 715 715 ========================================= Hits 2763 2763 ```

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