Option to enable brush rounding for centered bar charts

Currently brush rounding is silently disabled if bars are centered on bar chart as documented in #456. This is an intentional behavior implemented in response to #84, but can be a bit surprising.

This PR addresses the concerns of both #84 and #456 in a balanced (if slightly complicated) fashion by:

- Logging a warning to let you know that rounding when rounding was disabled due to bar centering
- Allowing you to override this behavior and round the brush anyways

Note that the volume and fluctuation charts in stock example are centered bar charts that look ok with rounded brush. This supports the idea that you should be able to enable rounding somehow on centered bar charts.