Fix group zone restriction handling

Now the group features (can_morph, can_pulse) are set after scanning the
features of all its members, if a member cannot do something then the whole
group will won't able to do that as well.
Enforce arguments and  color format for parsing

All colors must be 6 digits long and all commands enforce their specific
amount of arguments at command parse time.
Fix version
Fix import for Python 3
Refactor: Rename override_groups to cascade
Simplify cascading logic

Commands are now overridden in the order they were defined (either via a group
or single zone), making the logic more natural for end users.
Merge branch 'cascade-fixes'