New API that conforms to ROpenSci

Added

• NEW API: The bd_retrieve() family of functions will replace retrieve_data()
  soon. It consists of bd_retrieve(id = …), bd_retrieve_by_culture(collectionno = …),
  bd_retrieve_by_sequence(accession = …) and bd_retrieve_taxon(name = …)
  which is more auto-complete-friendly and should help avoid the internal need
  to sanitise inputs.

Deprecated

• retrieve_data() has been replaced as described above.
• retrieve_search_results() has been renamed to bd_retrieve_by_search().
• Both old functions will be removed in the next major version. Please update
  your BacDiveR-using scripts and other downstream code.