write_timestamp called during Dataset build should be exploded

[Gautzilla]

This function:

• Includes both timestamp operations and audio file validation functions
• Has some edgy features (.WAV and .FLAC extensions are tested only if there is no .wav or .flac extensions in the folder)
• Probably could only use pandas.Timestamp features instead of relying on regular expressions
• Should be split up in separate functions with both unit and integration testing, located in appropriate utils folders
• The test_create_timestamps.py associated test file should cover a larger scope