column name fix for merged statistics dataframe (zipchunks, postpro_node_merge)

When merging various statistics files from the zipchunks analysis type, the column names of the fatigue values were shifted one position in the wrong direction. Consequently, m=3 contained the m=4 values etc. For m=12, the intabs values where observed instead. This error occurred in dlctemplate.py because the merging of the various zipchunks was based on a hard-coded list of column names, and that list contained a typo. With this fix the column names are instead directly taken from the header of one of the chunks, and consequently removes this failure prone mechanism of hard coding the columns names.

All users of zipchunks and postpro_node_merge should update their local copies of dlctemplate.py (and run the latest version of the toolbox).

This MR also contains 2 commits with a minor fix (regarding automatic unique channel naming scheme of hawc-dll outputs) and addition of two more convenience methods.