Writing EventList with Observation.write set MJDREFI and MJDREFF to 0

[MRegeard]

When one write an EventList with Observation.write, MJDREFI and MJDREFF are set to 0 in the header of the EventList.
This obviously jeopardize the time of the written EventList.

This behaviour originates from updating the header of the EventList with pointing information without passing the reference time:

gammapy/gammapy/data/observations.py

Line 581 in 2475a00

events_hdu.header.update(self.pointing.to_fits_header())

Therefore,

gammapy/gammapy/utils/time.py

Line 25 in 2475a00

DEFAULT_EPOCH = Time(0, format="mjd", scale="tt")

is used.