[QST] Proper way to do mean centering for fixed-row-length LIST type column

In machine leaning practice, it's very common to calculate mean value and do mean subtractions for input column data. For example:

The input is a LIST type column data with fixed row-length of 3:
```
[1,2,3]
[3,1,2]
[2,3,1]
```
User wants to do mean centering for it: calculate the mean value for each sub column, in this case, it's always 2. Then do mean subtractions:
```
[-1, 0, 1]
[1, -1, 0]
[0, 1, -1]
```

Is their any nice way to do it? Currently I can only think of:
1. use `cudf::lists::extract_list_element` to extract all sub column
2. calculate mean value for each of them
3. do subtraction
4. construct them back to a LIST column.