Also warn when waveform files are being skipped on import due to this
kind of splitting.
The chunks YAML now emits all unique chunks found in files with the
same session ID.
Note that a single file can contain multiple chunks covering multiple
sessions. These will all be saved in the YAML file corresponding to the
original file's name, rather than the session ID encoded in any chunk.
This slight discrepancy is intentional, since the chunk YAML is meant
to test the parsers, to verify that they correctly decode a specific
input file. When importing data into a session, we use the actual
session ID specified by each chunk. Thus the session YAML files will
be derived from the proper chunks, regardless of their original
containing file. (Well, except for waveforms, but they don't appear
to have more than one session ID per file.)
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