Get ChEMBL webservice schema

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When I make a webservice query to ChEMBL some fields in the response are often NULL. Because the offline version of ChEMBL contains NA in these fields, and it is generally easier to work with NA-s in R, it makes sense to convert NULL-s to NA-s. But do we convert to NA_character_ or e.g. NA_integer_? It depends on the field, but the webservice query will not have this information. The solution for this is to somehow know which field has what class and assign the appropriate NA type.

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I found schema.json files (one for each resource) which contain this data. We can fetch these during each query and then use them to convert NULL-s to appropriate NA-s. Example here: https://www.ebi.ac.uk/chembl/api/data/molecule/schema.json