fix bug in checking a data file is multiplexed

Closes #223 (closed)

Also closes #224 (closed)

The different between data plotted for Centaur when mass position is selected and when it isn't selected is because Centaur is detected as non-multiplexed while it includes multiplexed files.

This MR fix bug not continue to add chans_in_stream to detect multiplex when the flag is_multiplex is changed to False.