Vaf interpreters pass per-library fields separately to reporters

This makes them function more similarly to per-sample fields.
This will allow us to more easily pass through additional informations (such as labels for the samples/libraries).
Yes - this description is pulling double duty - when used in the docs we don't have any actual library names, so it just gets a placeholder.  When it is used in a legend file for a report, there will be one column per library and they will have the actual library names filled in.