Ignore NaNs in transmit_type when determining BB vs CW channels #1578
Description
In some datasets, CW channels are transmitted in an alternating ping pattern, which results in transmit_type containing NaN values on every other ping. The current _get_chan_dict logic interprets “not all transmit_type == 'CW'” as meaning that BB data must be present, so the channel gets incorrectly placed in the BB group.
NaN entries should be ignored when determining whether a channel is CW or BB. If all non-NaN values are "CW", then the channel should be classified as CW.
Ref: #1577