More things for the future!

1. A better way to supply additional document-based stats

Currently, every time we supply an additional document-based stat (like doc length, unique terms, wdfdocmax), we have to change the API of Xapian::Weight incompatibly. This means any user subclass need updating (and conditionalise code to work with old and new versions of Xapian). This becomes uncomfortable for users who are affected. We need to come with a plan that supplies additional stats without changing the API.

2. Check that wdfdocmax and unique terms have correct values when passed to get_sumextra().

It will be good to have test cases to ensure that the correct value is being passed to get_sumextra().