Rework vignette to use CopernicusMarine

[lentinj]

"I'm writing about the mfdb package you have published on CRAN. I noted that in the vignette below, you use a discontinued service from Copernicus Marine, using a package that is not (or no longer) available from CRAN (RCMEMS):
https://cran.r-project.org/web/packages/mfdb/vignettes/import_hydrography_cmems.html

You may want to update this vignette with the https://cran.r-project.org/package=CopernicusMarine package, which still works and is actively maintained."