* insert.ring(): it is now possible to insert a ring before the first
  previously existing ring
* insert.ring() and delete.ring():
  - check for valid argument values
  - ensure that sequences used for indexing are increasing or zero-length


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/dplr@816 edb9625f-4e0d-4859-8d74-9fd3b1da38cb