Widgets bug fix and Active Template Model widget

This PR:
1. Fixes the bug reported in #493 . Specifically the `visualize_images` widget was crashing if the first image of the provided list didn't have landmarks.
2. Adds an extra RGB viewing option in the channel options (as proposed in #493 ). The checkbox only appears if the current image has 3 channels.
3. Adds `visualize_atm` widget that follows the structure of `visualize_aam`.