Add a script for generating hansard statistics for Jessica

Fixes #1160.  This was a one-off request, but probably worth
committing to the repository in case we need to regenerate these
in the future.  This is a very slow and inefficient script, since
it does multiple lookups for positions for each speech in the
date range; it would have been better to look up all of the
relevant positions in advance - however, this is a case where
trading off speed of development against speed of execution seemed
reasonable.