Fix raw bug -- open-with-config had the wrong function signature

Fix raw bug -- open-with-config had the wrong function signature, was missing a 'const' and therefore not properly overriding the base class.
Fixes #995.

Because of the propagation of constness, I needed to also alter some of the individual lookups from config (and in the process switched them from the find_attribute idiom to the more compact get_string_attribute idiom.