readcurr() upgrade

[mdsumner]

All the Copernicus altimetry is now more consistent with file and read functions for "adt/err/ugos/vgos/ugos/vgosa", so I'm inclined to change readcurr. We could remove the uonly/vonly args, point to read_ugos/vgos, and leave the magonly/dironly options.

I'd like to remove readcurr() from returning two layers, and being unable to return a time series.

So, maybe

• keep the behaviour but deprecate it
• create and point to read_mag_daily() and read_dir_daily() instead for magonly/dironly
• point to read_ugos/vgos when either of uonly/vonly are set

changes are in https://github.com/AustralianAntarcticDivision/raadtools/blob/gdal-new-world/R/copernicus_altimetry.R

• need to make sure the polar versions work the new way too
• fix up readcurr and currentsfiles
• check for other inconsistencies ...

[mdsumner]

now done in readcurr() 83c2991 it can still return 2 layers for one time step, but points to the new versions

More details are in the NEWs, where I'll collate learnings as we go.

will leave this open for extra checks, but read/extract/xylim all working as expected (we're handling the pacific/atlantic thing in the conversoin of fullname to vrt_dsn as a special case, later we'lldo that with "vrt://" and a look up (because no need to template a big string replace).)