This function loads the dataset record index from the JSON file in the SAFE data directory and sets which datasets are currently available and which records are the most recent available under a given concept.
load_index()Returns NULL invisibly - use get_index() to obtain the index data.
When any of the three index files (index.json,
gazetteer.geojson and location_aliases.csv) are loaded,
the file contents are cached in memory to reduce load times. The cache
is within an environment that is not exported in the package namespace
and is not intended to be user accessible.