IO_NC4_CHUNK_SIZE must allow 3 values for when dealing with cubes.

``gmt_nc`` has
```
size_t nc4_chunksize[3] = {0, 0, 0};
gmt_M_memcpy (&nc4_chunksize[cube], GMT->current.setting.io_nc4_chunksize, 2U, size_t);
```
but ``gmt_defaults.h`` sets only 2
```
size_t io_nc4_chunksize[2]; /* NetCDF chunk size (lat,lon) on output [0] */
```
and ``gmt_init.c`` only accounts for maximum two chunk values, but we may need 3 when dealing with cubes.