Update for IUCN API v4

[matthewharris02]

Updating the IUCN API to v4.

Main changes:

• New API has a less direct way of getting the habitat data, so I adapted the approach from the iucnredlist R package.
• Change the way of authorization from parameter to header in requests.get()
• Some key word changes
• Habitat codes in API now with _ not .
• Removal of region option

A test script is also included in the pull request.