unomment timestamp consistency checks

There are certain inconsistencies in timestamps in cha files: sometimes the next onset is not equal to the current offset, sometimes the offsets come before onsets. (see also [here](https://github.com/BergelsonLab/one_time_scripts/tree/8b61f810592189c7ad80e7e18a3fbdd3c001c201/20220124_inonsistent_timestamps_in_cha_files)).

I commented out the corresponding assertions in the `_extract_timestamps` function for now. They need to be uncommented or deleted once the inconsistenices have been dealt with.