Searches in Gnus are always conducted on one or more groups. Right now,
using the notmuch search engine, it's not possible to turn those groups
into path: search terms in the notmuch query, though that's clearly the
sensible thing to do (it looks like rudimentary support for path
filtering was halfway added, but not quite).

This patch creates a new `nnir-notmuch-filter-group-names-function'
custom option which transforms group names into something notmuch is
likely to understand.