adjusted changelog module for easier unittesting

- Removed instance of write_changelog_entry() that was not being
  used bc duplicated.
- Edited write_changelog_entry() to accept timestamp arg.
- Changed var name for clarity.
unit tests for DDR.changelog
unit tests for DDR.__init__.py

Note that Timer is not tested bc not complete and not used.
increase entity.alternate_id field maxlength bc NARA