beautify channel representation

[sorlob]

Currently channels are represented with there memory addresses in contrast to labels.

[<vitabel.timeseries.Channel at 0x7f619be3ba70>,
<vitabel.timeseries.Channel at 0x7f619be3bbb0>,
<vitabel.timeseries.Channel at 0x7f619be3be30>,
<vitabel.timeseries.Channel at 0x7f619be58050>,
<vitabel.timeseries.Channel at 0x7f619be58230>,
Label(Inspiratory Time),
Label(Expiratory Time),
Label(Respiratory Rate),
Label(Maximal Inspiratory Airway Pressure),
Label(Minimal Inspiratory Airway Pressure),
Label(Delta VT),
Label(VTinsp),
Label(VTexp),
Label(VTinsp_cum),
Label(VTexp_cum)]