feat: timestamp unit specification

[jakobchwastek]

Description

Fixes issue #626

Implemented changes

• Introduced support for seconds, milliseconds and steps as units of timestamps in the time_column

• Added a time_unit parameter to the constructor of GazeDataFrame

• Add a time_unit attribute in the DatasetDefinition class

• Developed a private function in GazeDataFrame to convert input timestamps to milliseconds.- Expand functions gaze.from_csv, gaze.from_pandas, gaze.from_numpy to include a time_unit parameter

• Incorporated time_unit into existing local dataset definitions

• Updated tests and documentation examples to reflect these changes.

A time_unit specification is mandatory when a time_column is included in the data. To ensure compatibility with certain event detection methods (e.g. idt) I choosed to cast time values to integer after conversion to millis when possible. For example, float timestamps in seconds with millisecond precision (e.g., [0.001, 0.002]) are converted to integer milliseconds ([1, 2]).

Type of change

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

How Has This Been Tested?

• Enhanced existing tests: Updated tests that utilize time values in GazeDataFrame to include the specification of the time_unit.
• Added conversion tests to the GazeDataFrame init tests.

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 (6a3af88) to head (09f4a20).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #643 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 57 57 Lines 2669 2696 +27 Branches 688 695 +7 ========================================= + Hits 2669 2696 +27 ```

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