[ecco-support] [EXTERNAL] grid_load.m

[Zhang, Hong (US 398K)]

Hi Gael,
I took a look. We need to modify
grid_load.m and read_nctiles.m for r4.

"read_nctiles.m" needs two minor fixes
1) https://github.com/MITgcm/gcmfaces/blob/master/gcmfaces_IO/read_nctiles.m#L64
basedir = './'; ==> basedir = '.';
2) https://github.com/MITgcm/gcmfaces/blob/master/gcmfaces_IO/read_nctiles.m#L190
    t0=tt(1)-1; ==>     t0=tt(1);

It's hard to reconcile "GRID*.nc" from r3 and "ECCO-GRID*.nc" from r4
in "grid_load.m" because they have different field names.
One work-around is to copy 
https://ecco.jpl.nasa.gov/drive/files/Version4/Release3/nctiles_grid/GRID.nc
to
https://ecco.jpl.nasa.gov/drive/files/Version4/Release4/nctiles_grid/

I did a test as above and it's OK.
But it took ~204 seconds to load 312 monthly v4 ETAN from 312 files
as opposed to 1.4 seconds to load 288 monthly v3 ETAN from 13 files.
Perhaps this arrangement has more benefits to ECCO-v4.py

cheers
Hong