Fix chembl_query() for the compound structural alerts resource

[stitam]

Related to #443.

This PR fixes access to the ChEMBL compund structural alerts resource. I am unsure whether it was the implementation that was wrong or the webservice has changed. In the previous implementation the required input for these alerts was a compound structural alert ID (so the ID of the alert) but it doesn't work. Instead, the webservice requires a compound ChEMBL ID. This PR fixes this. Also the resource uses an URL that is different from most ChEMBL resource URLs. This PR updates the resource URL as well.

Note, when we query ChEMBL, most resources return a json which is parsed as a nested list. In most cases this can be simplified, e.g. when list elements are columns of a data frame, we can replace them with a data frame. We do these simplifications automatically, but I realised for the structural alerts resource the simplification strategy is not trivial. Because of this I decided to add a new argument tidy which is TRUE by default and governs whether we should attempt to simplify the output or just return the nested list. I hope this does not add to much complexity and will allow us to experiment with better tidying strategies later.

PR task list:

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