Don't replicate govuk page statistics by default

This is a really big dataset and is rarely need, so by default we should
not pull it down.
Improve perf characteristics of filling gaps

Previously for every period and expected permutation of group_by key
value it would loop over all data points trying to find a match. This
led to num_periods * num_permutations * num_data_points iterations,
which will get very slow very fast. We will always need to go through
every period and permutation but removing the last multiple has big
change.

I switched this for pre-hashing the data points so that we can find a
match in O(1) rather than O(num_data_points). The reduced the time to
fill in gaps from 180s to 0.175s for the troublesome lpa journey query.

I have ensured that the output of this function is the same. The pickled
arguments and results are pulled out by wrapping
fill_group_by_permutations in a decorator that grabs *args and **kwargs
and the results and stores them in a file.

You can see the result of the profile by running:
	`nosetests --nocapture tests/perf/`
Merge pull request #371 from alphagov/fill-data-perf-improvments

Fill data perf improvments