Switch over included datasets to download via `retriever`

[ha0ye]

There are issues open in https://github.com/weecology/retriever to add the:

• Cowley herps data - Add Cowley herp data retriever#1424
• South African National Parks data - add SanParks data  retriever#1421

Once retriever has working scripts for those, we need to remove the files from inst/extdata, and modify the functions in get_cowley_data.R and get_sanparks_data.R:

• use retriever as the source of data
• extract any remaining metadata into the appropriate fields

[ha0ye]

Current status:

• updating retriever script for South African National Parks to use the DataOne links that @mbjones provided
• keep Cowley herps data as part of MATSS, but update metadata to acknowledge original source and redistribution from the publication (does that sound right? @gmyenni )