Add example for selecting a subset from a backed adata

[sgratiy]

Problem:
View of backed data does not allow adding annotations.
For example something like this would work for AnnData and View of AnnData in memory:
normalized_adata.uns['hvg']={"flavour":"hicat"}
but this fails for a View of backed data

Solution:
Explain users how to use AnnData when working with the subset of backed data by adding to the notebook the following:

# if wanting to work with a subset of data in backed mode (here choosing the first 1000 cells):
normalized_adata = normalized_adata[:1000,:].copy(filename='./data/normalized_subset.h5ad')
print(normalized_adata)

apparently this is by design. See the very end of the anndata blog post.
Validation:
HVG on a subset done this way does not fail:

Note:
if we follow this route, we should also add nice exception message if users do pass View to functions. Not sure how many modules are affected by this.