Add EOT20 global tide model

[leuliett]

Hart-Davis Michael, Piccioni Gaia, Dettmering Denise, Schwatke Christian, Passaro Marcello, Seitz Florian (2021). EOT20 - A global Empirical Ocean Tide model from multi-mission satellite altimetry. SEANOE. https://doi.org/10.17882/79489

https://essd.copernicus.org/preprints/essd-2021-97/essd-2021-97.pdf

[leuliett]

The data is available at SEANOE: https://doi.org/10.17882/79489, with a publication on the data being available at the Earth System Science Data journal: https://doi.org/10.5194/essd-13-3869-2021.