Third pass to further refine data structures and enhance functionality.

With this commit, we add metrics-generation functionality to the
Norris utility.  Issues searched ("nrs-search") and extracted
("nrs-extract") can now be analyzed ("nrs-analyze") for structure
and flow metrics.   Metrics can be converted into simple textual
format ("nrs-m2txt") or to CSV for import into Excel ("nrs-m2csv").

Note that the CSV conversion is fairly primitive right now, designed
mainly to give Mike Simpson something he can easily kludge into his
metrics spreadsheet.  There are a couple of other warts in various
places; once this code has been in real use for a bit, it's probably
worth another pass.

Also note that this commit adds some rudimentary Python docstrings to
all of the class definitions, in the interest of being able to remember
why things were done a certain way a couple of months down the road.