Download ChEMBL schema files once per session

[stitam]

Related to #449.

There was an inconsistency issue between webservice requests and offline queries: when a value was missing from the respective database, the webservice often returned NULL but the offline query returned NA with the appropriate class (e.g. NA_character_). In PR #450 I implemented a fix in which we download a schema file from the ChEMBL resource and use that to replace NULL values with the appropriate NA values. However, downloading the schema with each query is inefficient.

This PR implements session level caching. When we load the package we create a new environment chembl_schema_cache. Whenever we need the schema file for a resource, we look for the schema in this environment. If we find it we use it, otherwise we download it and assign to the environment.

PR task list:

• Update NEWS
• Add tests (if appropriate)
• Update documentation with devtools::document()
• Check package passed