D3D updates (#428)

Updates to `io/d3d.py` code 

- Added new coordinate names for the latest version of Delft3D
- new `calculate_grid_convergence_index` function to calculate the grid
convergence index between two grid's results basted on the equation
[GCI]( https://www.grc.nasa.gov/WWW/wind/valid/tutorial/spatconv.html)
- Allows xarray or netCDF4 input

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Co-authored-by: akeeste <akeeste@sandia.gov>
Co-authored-by: Simms, Andrew <andrew.simms@nlr.gov>

Author: 3 people