flood of warnings when running tc_tracks_forecast.fetch_ecmwf

[emanuel-schmid]

When running TCForecast.fetch_ecmwf a very high number of warnings are delivered, like so:

...
~$ climada_petals\climada_petals\hazard\tc_tracks_forecast.py:515: UserWarning: Converting non-nanosecond precision datetime values to nanosecond precision. This behavior can eventually be relaxed in xarray, as it is an artifact from pandas which is now beginning to support non-nanosecond precision values. This warning is caused by passing non-nanosecond np.datetime64 or np.timedelta64 values to the DataArray or Variable constructor; it can be silenced by converting the values to nanosecond precision ahead of time.
  track = xr.Dataset(
~$ climada_petals\climada_petals\hazard\tc_tracks_forecast.py:515: UserWarning: Converting non-nanosecond precision datetime values to nanosecond precision. This behavior can eventually be relaxed in xarray, as it is an artifact from pandas which is now beginning to support non-nanosecond precision values. This warning is caused by passing non-nanosecond np.datetime64 or np.timedelta64 values to the DataArray or Variable constructor; it can be silenced by converting the values to nanosecond precision ahead of time.
  track = xr.Dataset(
...

To suppress these for the time being all UserWarnings are caught and ignored in _subset_to_track, see f89e496.

[emanuel-schmid]

see similar issue #84.