Added istransiting tag to transiting planets

All transiting planets now have `<istransiting>1</istransiting>` although a comparison to other databases should be done. This is the method decided by #31.

The cleanup script now marks all planets detected by the transit method or that have a transit time with this tag. If a planet has a radius measurement and is not an imaged planet it is raised as an issue by cleanup unless added to the exclude list.