BNF vs BNF legacy

[richiecroker]

On openprescribing.net we refer in a number of place to "BNF" - whereas in fact the hierarchical system we use actually refers to the old BNF Legacy system, as this is what the BSA still use for the English Prescribing Dataset.

We need to make it clearer throughout the site that when we say "BNF" we may mean "BNF Legacy". This includes in references in pages, and probably a new section in the FAQs.