make externallinks for datasets work (reading)
make externallinks for datasets work (reading)

refs #274
this change is transparent on the API level

that means there is no API method to query the external link
status for datasets and there is no gold plated way of creating
link to datasets, but all that is required is a dummy dataset
with the napimount attribute set

git-svn-id: 1001b6e0dd469eaa302a741812f7f5d223cf35f7@1594 ff5d1e40-2be0-497f-93bd-dc18237bd3c7

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