add (failing) test for isexternaldataset
add (failing) test for isexternaldataset

refs #345

git-svn-id: 85fbbed3e45021658c4e227f979e041b2c214cca@1881 ff5d1e40-2be0-497f-93bd-dc18237bd3c7

chrisemblhh/nexus,chrisemblhh/nexus,nexusformat/code,chrisemblhh/nexus,nexusformat/code,nexusformat/code,chrisemblhh/nexus,nexusformat/code,chrisemblhh/nexus,nexusformat/code