Add tag hierarchy

[sflis]

Having at least two levels of tags to create tag groups would be useful as quite a few tags share common features. For example Mrk501 Mrk421 PG1553 are all tags for observation so they could be grouped under the observation tag like so:

observation:Mrk501, observation:Mrk421, observation:PG1553

However this might be done using the proposal of #5 by or-ing the tags you want to group and create a new tag.