Utility function used by get.acs().
Details
Interprets as 'bg' any of these:    150, '150', 'blockgroup', 'block group', or 'bg' (or variants, ignoring case) 
Interprets as 'tract' any of these: 140, '140', or 'tract' (or variants, ignoring case) 
Interprets as 'both' any of these: 'both' or a vector that has both of the above terms (or variants, ignoring case).
See also
get.acs() which uses this