Replace `getFAO` with `read_fao_data`

This issue is a followup to #12 (closed). The function needs to be replaced with one that uses the new APIs.

The change will lead the the following differences in functionality

Gains

• Support for multiple elements
• Depends on up to date packages
• Will actually work

No change

• Still won't work for domains that don't have te classic 4 dimensions

Losses

• Loss of CHMT China fix (have to figure out how to use it when user asks for M49 country codes