Checkpoint. Change initial CSV values to None.

- Improve misc code, variable names and comments.
Misc variable name & comment improvements.
Complete and test write & read for stats CSV files.

- Add code to determine number of unprocessed images for today and
  previous days.
  - Add test to write, read back and verify datecam tables.
  - Fix bugs in code and comments.
Fix and test minute_stats tally of unproc'd files.
Add "changed" flag to datecam table.
Change arguments of proc_stats to just a file path.

- Update test000proc_stats() accordingly.