encode with i4 dtype

[patrickjwright]

This addresses comments from Øystein at Norwegian Met Service, that long int (int64) dtype is incompatible with OPeNDAP access on the THREDDS server.

Are there any concerns that this will break anything else using the .nc files?

The year 2038 problem appears to be a real concern, but I suppose we don't worry about that now! (And hopefully down the road THREDDS and/or OPeNDAP will upgrade to support this).

[PennyHow]

I just checked this and it all works fine. NetCDF output files can be loaded as follows with no problem:

import xarray as xr
data = xr.open_dataset(<NC_DATAFILE>)
print(data.time)

I hope some solution for the Epochalypse problem surfaces before 2038!