Merge: Remove "Level" in "xxxLevelStatistic" [ following the Hackathon ]

It has been agreed previously to rename `[Peak|Cluster|Set]LevelStatistic` in `[Peak|Cluster|Set]Statistic`. During the HBM Hackathon we found out that this change had not been propagated to all files (especially FSL examples used in testing).

This pull request homogenise the naming to remove `Level` everywhere a mention is made to a `[Peak|Cluster|Set]Statistic`.