Implementing copy constructor for channel objects

[fschlueter]

Currently to copy data from one to the other channel, we execute code like:
new_channel.set_trace(orig_channel.get_trace(), orig_channel.get_sampling_rate()
It can happen that the following line is missed (although it is critical):
new_channel.set_trace_start_time(orig_channel.get_trace_start_time())
We should implement an operator or function to copy all relevant information with one line.