Asig.cn and Asig.col_names are confusing and unnecessary to have both

[wiccy46]

Asig.cn is a list of channel names, e.g. ['Left', 'Right', 'Center'...] , and then there is another member call Asig.col_name that is only used in get_item to help create a sublist of self.cn based on channel subset.

I think self.col_name is not necessary or at least it shouldn't be exposed.