feat: support 2kHz ASC files with floating-point timestamps

[saeub]

Description

EyeLink's edf2asc CLI has an option -ftime that will output timestamps as floating point numbers. For 2000Hz recordings, this avoids issue #688, except that parse_eyelink() doesn't support floating-point timestamps. This PR adds this support.

Related to issue #688 (doesn't fix it, but might make it redundant?)

Implemented changes

Insert a description of the changes implemented in the pull request.

• [x] Add floating point digits to regex patterns in parsing.py
• [x] Use Float64 dtype instead of Int64 for timestamps in DataFrames. (Note that for <= 1000Hz, the timestamps will still be converted back to Int64 when calling from_asc(), so there is no change of behavior for from_asc() with other sampling rates.)
• [x] Necessary int→float conversions to make the type checker happy.

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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] Added a test case to asc_test.py::test_from_asc_has_shape_and_schema with a new test file eyelink_monocular_2khz_example.asc (created by hand)
• [x] Tested locally with a real ASC file generated with edf2asc -ftime

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 (58b1863) to head (ec71a3b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #739 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 60 60 Lines 2763 2763 Branches 715 715 ========================================= Hits 2763 2763 ```

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