WIP: Consistency between label and type (including renaming of xxxStatistic in xxx)

This pull request first include a couple of fixes to have consistent `prov:label` attributes given the `prov:type` of an entity, in particular:
- `prov:label` of `SearchSpaceMap` becomes "Search Space <b>Map</b>" (instead of "Search Space")
- `prov:label` of `SubVolumeMap` becomes "Sub-Volume <b>Map</b>" (instead of "Sub-Volume")

Then, in the same spirit, as discussed at the TF meeting in Hamburg with @nicholst, @gllmflndn and @jbpoline: `PeakStatistic` and `ClusterStatistic` have their type updated to `Peak` and `Cluster`. 

As `SetStatistic` should be renamed in `Set` for consistency, I would like to propose to have the `SetStatistic` attributes moved to the `ExcursionSet` entity. This way we have a hierarchy of `ExcursionSet`, `Cluster` and `Peak` and each of them has attributes related to the statistic associated with it.