Optional printing of data value in popover

The data value of a bubble was always printed in the popover. We needed to disable the printing of the data in the popover because the absolute values were not relevant for a few of our graphs. This was caused by the absolute values be approximations. This pull request introduces a key in the popoverOpts configuration dict with the name popoverShowData which can be set to false to disable the printing of data in the popover.