Use `chembl_downloader` for generating substructure library

[cthoyt]

The chembl_downloader automates downloading the ChEMBL SDF dump, and there's a little code that wraps gunzipping it and creating a ForwardSDMolSupplier in a context generator. This will hopefully make it a bit easier for someone else to use this code!

The updated notebook can be viewed on the NBViewer app at https://nbviewer.jupyter.org/github/cthoyt/rdkit_blog/blob/update-notebook/notebooks/Generalized%20Substructure%20Search.ipynb.

I wrote more about it on my blog https://cthoyt.com/2021/08/04/taming-chembl-sdf.html - this package could be the subject of a lightning talk at a future RDKit UGM for enabling more reproducible use of ChEMBL

[cthoyt]

Hi @greglandrum, any chance you will consider this?