Use SparseTensors in preprocessing

[tivaro]

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The SparseTensors implemented in NGF/sparse.py are very well suited for the molecule data structures/distributions used in these repo. An advantage of the SparseTensors tensors is that they do not require padding.

It would be easy to implement an append or an extend method to merge the tensors. This would significantly speed up the preprocessing for larger datasets, and obsolete the padaxis and concat_mol_tensors functions.