Small pour detection

Small pour detection was present in kegbot-pycore until commit 7711c88

https://github.com/Kegbot/kegbot-pycore/commit/7711c889428b96c6ad44e293028377a38241c15f#diff-93c7270b970dd3359f27e37b343aa6c7L384

This pull request will add back in the small pour detection. The goal of commit 7711c88 was to decouple flows from taps. Sadly to add back in small pour detection I needed to pass a Tap object into the AddTicks method on the Flow object. However if no Tap object is passed, it will simply not do minimum flow verification.