Add heat budget tutorial

[jetesdal]

I like to submit a first draft of the requested tutorial to close the global heat budget in ECCOv4. It is an extension to the heat budget calculations presented in ecco_budgets.ipynb, now including the global ocean. The code works with the netcdf files from the PO.DAAC Drive. The outline and explanations are added to follow a similar format as Volume_budget_closure.ipynb. I tried to do everything with lazy evaluation using xarray and dask, but there are a few cases where calculation load the data into memory. Maybe @rabernat might know how to avoid this. @ifenty I'm open to any suggestions.

[ifenty]

@jetesdal Your heat budget tutorial looks really nice. There is an easier way to load the geothermal heat flux field (really any field saved in the llc format) using the routine 'read_llc_to_tiles'. I made a short notebook to demonstrate on this temporary hosting site. Check it out.

Here is the 'read_llc_to_tiles' code in action and an example of how I convert the geoflx ndarray to xarray DataArray.

geoflx = ecco.read_llc_to_tiles(input_init_dir, 'geothermalFlux.bin')
geoflux_llc = ecco_grid.XC.copy(deep=True)
geoflux_llc.values = geoflx
geoflux_llc.name = 'geoflux'
print(geoflux_llc.dims)
print(geoflux_llc.name)
geoflux_llc.attrs['name'] = 'geothermal flux'

After you implement the use of 'read_llc_to_tiles' I think we can merge.

[jetesdal]

Thanks @ifenty for the tip. The tutorial is now using 'read_llc_to_tiles' to read the geothermal heat flux field. When converting to xarray, I suggest using xr.DataArray() instead of ecco_grid.XC.copy(deep=True). Otherwise, you'll get attributes that don't make sense (see screenshot).

[ifenty]

@jetesdal agreed. Looks good!