Fetch Uniprot entries for a list of organisms

[nanglo123]

This PR adds a method to retrieve Uniprot entries (download_uniprot_entries_for_organisms) for a list of organisms

• The method was implemented primarily to retrieve unreviewed + reviewed Uniprot entries; however, users can also fetch only reviewed entries
• Users specify the column names they want retrieved, the valid fields are listed here and the list of organism ids (NCBITaxon IDs) they want to extract entries for
• This was added as a separate method rather than to existing functionality because download_uniprot_entries has a fixed output format that a parameterised version would break
• The downloaded file is stored at a user supplied path.