[x] Simplify choosing data dimension and data type between data_dim and grid_dimensions, data_dtype and field_dtype.
[x] If data_ptr = NULL, libyt shouldn't abort when data_dtype or field_dtype is not set and check_data==false. We don't need to wrap this array, hence no need to set data type.
Update
libytMPI_Gathervnot support send count > INT_MAX.data_dimandgrid_dimensions,data_dtypeandfield_dtype.data_ptr = NULL,libytshouldn't abort whendata_dtypeorfield_dtypeis not set andcheck_data==false. We don't need to wrap this array, hence no need to set data type.append_grid.cpp.