This is a helper to allow srvyr's syntactic style. In particular, it tells
functions inside of a summarize call what survey to use. In general, users
will not have to worry about getting (or setting) the current conext's survey,
unless they are trying to extend srvyr. See vignette("extending-srvyr")
for more details.
current_svy()
a tbl_svy (or error if called with no survey context)