Change read_mseed_channels to use MSeed's RecordReader and apply is_multiplex

Replace read_mseed_chanids_from_header() which is based on Obspy by _read_mseed_chanids() which uses MSeedRecordReader to read channel header and is_multiplex to skip reading multiplex records.

This MR uses is_multiplex return from detect_data_type which comes from !138 (merged). So we can only run this after !138 (merged) is merged.