FR3D annotations of RNA basepairs updated

As new RNA-containing 3D structures are solved, we have more examples of the many categories of RNA basepairs, across the 12 Leontis-Westhof families.  Some new instances fall just outside the cutoffs used by FR3D for basepair annotation.  In late 2016, we re-examined each basepairing category and re-set the cutoffs to more accurately capture the instances which meet the geometric criteria described in the papers of Leontis, Stombaugh and Westhof from 2001 and 2002.  In addition, we improved the calculation of one parameter, the angle of rotation of one base in the plane of the basepair, which allowed us to more accurately categorize certain basepairs.  New annotations are now posted for all RNA-containing 3D structures.