Change format in which event timestamps are written to HDF5 files.
Instead of a single float64 MJD value which has ~µs precision,
the CTAO high precision time format is used now.
This stores the timestamp as two uint32 values: seconds
and quarter nanoseconds since 1970-01-01T00:00:00.0 TAI.
This only affects the storage format and the precision,
the in-memory API is unchanged as it relies on astropy.time.Time
and values are converted when reading/writing from/to HDF5.