Restructure data directories in NSHM repos

Parent issue. Create supporting issues as needed.

In nshm-conus and nshm-alaska:

• Create a new data/ directory
• Copy site-data/ to data/site/ (won't delete original until ModelLoader refactored)
• Move catalogs to data/catalog/
• Create data/gps/ and add appropriate GPS file formerly in nshmp-ws
• Add data/README.md

In this repo:

• refactor site-data location (pmpowers)