* add skeleton to retrieve zigbee groups
* get single group
* add a group
* return group members with group
* add comment
* fix group members
* add function to add device to group
* add group members
* add remove from group method
* add api to remove members from group
* add remove groups method
* clean up group add and remove
* fix remove group
* fix remove groups
* add api to get only groupable devices
* change var init
* add tests
* address review comment