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 (cf. issue #68) 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`.