Hope to add multi-dimensional slicing function for VGroup like Numpy.Array

Nobody has claimed this yet.

## Description of proposed feature

A slice function like Numpy.Array,for example:

a = VGroup(Vgroup(..), ...) # 2 dim vgroup
a[1:4, 2:4] # slice

## How can the new feature be used?

Slice function will make great sense for 2 dim mobject vgroup，if 2 dim Square vgroup represents a image, I want to get a part of image(that is a patch)，but there is no simple method to impIement it, I have tried many times to overwrite Vgroup's method __getitem__（succeed to Group），but there is bug, which will affect other functions.

## Additional comments

Start with VGroup.__getitem__ and compare it with the existing Group behavior described in the issue. Reproduce the two-dimensional example a[1:4, 2:4] and investigate the reported effects on other functions. Done means multidimensional slicing works for nested VGroups without breaking existing indexing behavior.

Written by the indexing model from the issue text.