Split listCensusMetadata() into list_variables(), list_geographies(), list_values(), list_groups()

[hrecht]

Split metadata retrieval function listCensusMetadata() into list_variables(), list_geographies(), list_values(), list_groups().

When the package was first written, only simple geography and variable metadata existed. The current mega-function is now overly complicated for users, unwieldy and inflexible for development, and not future-proof.

Separate functions will be much simpler to use and provide better functionality.