Add dummy CO atom type so we can read the RMG-Java database.
Add dummy CO atom type so we can read the RMG-Java database.

We cannot interpret or understand the RMG-Java database as we 
don't know what this atom type means. This is just so that we
don't throw an exception when I'm trying to use the rmg.structure
module as a dependency in my database processing script.
   -- Richard
faribas/RMG-Py,faribas/RMG-Py,enochd/RMG-Py,KEHANG/RMG-Py,enochd/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py,KEHANG/RMG-Py,nickvandewiele/RMG-Py,nickvandewiele/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py,comocheng/RMG-Py,nyee/RMG-Py,nyee/RMG-Py,comocheng/RMG-Py