Separate render labels and render legend

Now two seperate options
Change group_label to group for consistency
Allow passing a single color
Make the landmark group vs label clearer

Use : instead of _
Update docs for new kwargs
Change render_labels to render_numbering

Update widget to allow you to toggle it on and off
MeanInstanceLinearModel.mean is now a method
whoops, should actually remove to @property...
Merge pull request #485 from jabooth/mean_as_method

MeanInstanceLinearModel.mean is now a method
move the other expensive PCA properties to methods
Merge remote-tracking branch 'upstream/master' into mean_as_method
Merge pull request #487 from jabooth/pca_methods

Change expensive PCA properties to methods
expensive image properties moved to methods
Merge remote-tracking branch 'upstream/master'
Merge pull request #488 from jabooth/image_properties

Move expensive image properties to methods
Fix bugs introduced by show_numbering

Seems to work for all widgets now
Merge pull request #486 from patricksnape/image_vis_improvements

Image visualisation improvements