read channels from header only

From reading data with blockettes that have no reader implemented yet, I see that reading channels currently goes too deep to the data while channels' information is in traces' headers.

This MR implements mseed_header.read_channels_from_hdrs() and processing.read_mseed_channel() to read channel ids for SOH Channel Preferences dialog and remove read_chan_only from mseed because read_mseed_channel is now independent from mseed.

read_channels_from_hdrs() scan through all traces to not miss any channel ids, which fixes the bugs due to consuming that files start with SOH traces have no waveform traces and vice versa.

Displaying tracking info feature is added to SOH Channel Preferences dialog as well.