storing dict in .h5ad should raise warning when keys include '/'

[MxMstrmn]

I was not aware how .h5ad stores their dicts and it took me quite a bit to figure out why my stored adata.uns['key'] was different from the original adata.uns['key']. Part of the problem was the large dataset which made examination of potential fail cases difficult.

Eventuelly, I figured out that some molecular descriptors include the sequence '(+/-)' which caused anndata to store the dict in a nested structure. I would suggest to check if keys include '/' and raise a warning such that the user is aware that the stored dictionary will not be identical to the original one.

Minimal Code example that clarified the problem to me:

import scanpy as sc 
from anndata import AnnData 

adata = AnnData()
bucket_list = {
    'remember': 'trivia',
    'forget/whatIwantedtoremember': 42, 
}
adata.uns['bucket_list'] = bucket_list
sc.write('adata_test.h5ad', adata)

adata = sc.read('adata_test.h5ad')
adata.uns['bucket_list']

 {'forget': {'whatIwantedtoremember': 42}, 'remember': 'trivia'}

[grst]

Would be better to fix this, rather than just raising a warning. Possibly by escaping/unescaping / in dict keys on save and load, but maybe there's a better solution.

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