Cast numpy datetime64s to [ns] before annotating xarray, to avoid warning

[dylanhmorris]

Cast numpy datetime64s to [ns] before annotating xarray, to avoid warning

tests/test_add_time_coords_to_idata.py::test_validate_input_types_in_add_time_coords_to_idata_dimensions[idata1-groups1-variables1-dimensions1-None]
  /home/runner/work/forecasttools-py/forecasttools-py/forecasttools/idata_w_dates_to_df.py:175: 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.
    idata_group = idata_group.assign_coords({dimension: interval_dates})