ClueGenerator: Do not produce Clues from PP constituents

The PP constituents typically still contain useful subconstituents from
which we can generate useful queries and introduce a lot of redundancy
which then skews passage match scores.
doc/TODO: A note about type system cleanup
PassByClue: Fix penalty for distance from beginning - was always 0 before

We also make it quadratic to even more emphasize first sentences.
PassByClue: Further emphasize the distance penalty - make it multiplicative