Remove state from unstructured data and IO functions

[dawe]

Since all annotation levels are stored, we can remove the need to keep gt.NestedBlockState in adata.uns['schist']. Once annotations are present, a new state can be easily reconstructed, given that

• the graph can be built from connectivities
• the block state can be built from nsbm_level entries in adata.obs

This can be done if we make sure that all parameters are stored in the appropriate dictionary.
If this is implemented, there is no need to use schist.io functions to read/write as they are there only to dump the state into a separate pickle

[dawe]

This has been addressed in v0.7.10. Unfortunately I realized there's a bug now with nested models as you cannot place lists of lists in adata.uns

[dawe]

Fixed in v0.7.11