Create helper function for each table available in `get_aux()`

[tonyfujs]

The get_aux(table = NULL) function call returns all available auxiliary data tables available for download.

It would be nice to have one helper function for each available table, similar to the existing get_countries() and get_regions() functions. It will:

• Be a nicer interface for end-users
• Allow to add some background information about each table in the corresponding function documentation