Checksum computation overhaul

Checksum computation overhaul

This adds advanced debugging of checksum computation.

Note: good sample data is lacking.  I think most of the actual
checksum lines included in the test are questionable.  The only
one that computes accurately seems to be the example from the
3M SIP Developers Guide that specifically addresses checksum
computation.  The others from the same guide seem invalid!

I think the debugging output suggests the implementation is
consistent with the specified instructions, despite unavailability
of sample data to confirm.