More fitmultilevel tidying

Few small improvements:

1. `menpo.fitmultilevel.validate_features` -> `menpo.fitmultilevel.checks.check_features` for consistency
2. `menpo.fitmultilevel.functions.mean_pointcloud` moves to and replaces `menpo.shape.groupops.mean_pointcloud`
3. Small docs improvements

Keeping these PRs small so they are easy to check - It would look daunting to drop all these changes in one PR, but they are actually fairly safe improvements.