CHANNELNAME2PROBENAME - convert a Marder channel name to a probe name
[PROBENAME, PROBEREF, SUBJECTNAME] = CHANNELNAME2PROBENAME(CHNAME, SUBJECTS)
Given a channel name (e.g., 'DGN1_A','lvn','lvn2'), returns a probe name
and subject name. PROBEREF is always 1.
If there is more than one subject (usually a maximum of 2), then the
program looks for a '1' or '2' in CHNAME. If none is found, then it is
assumed there is only 1 subject and 1 is the end of the string.
If a 2 is found and there is no second subject, a warning is produced.