Change landmark group indexing

Now when you index into a landmark group it returns the sliced
pointcloud instead of a new landmark group. This removes
a lot of spurious lms calls throughout the code.
Merge remote-tracking branch 'upstream/master' into landmarks_fixes
Remove group label from landmark group

This wasn't being handled properly and actually doesn't make much
sense. This group label was basically caching the key into
the landmark manager which it now no longer does!
Fix the documentation in the labellers
Refactor error messages in the labellers
Add the ability to only view specific groups/labels

This makes viewing landmarks easier with the new layout.
Now you can pass in group_label, without_labels and with_labels.
Update base.py

Super minor doc change
Merge pull request #402 from patricksnape/landmarks_fixes

Landmarks fixes