Ernst rain per month

Fix for https://github.com/nens/lizard-nxt/issues/323

See:

http://localhost:8000/#location=52.1251,5.3174,8&start=Nov-20-2013&end=Feb-23-2014&layers=topography,waterchain,demo:radar

It fixes the "bug"... But if you zoom further out you will have the same problem when looking at multiple decades. The RFC https://github.com/nens/lizard-nxt/issues/295 should take this into account.