add Antarctic altimetry

[mdsumner]

now in raadfiles::altimetry_antarctica_files

Data Source (1): dt_antarctic_multimission_sea_level_uv_20130401_20190731.nc ...

Grids (3) <dimension family> : <associated variables> 

[1]   D2,D1,D0 : sla, formal_error, U, V    **ACTIVE GRID** ( 283342500  values per variable)
[2]   D2,D1    : latitude, longitude
[3]   D0       : time

Dimensions 3 (all active): 
  
  dim   name  length   min   max start count  dmin  dmax unlim coord_dim 
  <chr> <chr>  <dbl> <dbl> <dbl> <int> <int> <dbl> <dbl> <lgl> <lgl>     
1 D0    time    2313 23101 25413     1  2313 23101 25413 FALSE TRUE      
2 D1    x        350     1   350     1   350     1   350 FALSE FALSE     
3 D2    y        350     1   350     1   350     1   350 FALSE FALSE