Update ant.py, node.py, channel.py, scan.py

[pirower]

Update ant.py Add of functionalities to unassign a channel in function unassign_channel Add of message IDs for unassigning a channel and closing a channel in function _worker

Update node.py Change of node variable channels from "list" to "array". At initialisation an array of the length of maximum number of possible channels is generated using the "get_capabilities" funciton Introduction of the function "get_capabilities" by using the "Message.ID.RESPONSE_CAPABILITIES" to get the capabilities of the ant+ chip. Add of functionality of threading of 'node'.

Update channel.py Add of functionality to unassign a channel in function _unassign

Update scan.py Proper closing and unassigning of the channel for scanning.