minor improvement of displayed error information by dj.AutoPopulate/populate_
prohibited nulls for integer attributes.  A few cosmetic changes.
added .gitignore, fixed a small bug in dj.GeneralRelvar/display introduced in last commit.
made dj.Schema/reload compatible with a future fix for mym's issue #2. Fixed a table declaration parsing bug introduced in 4cba4bd.
fixed a bug that (under rare circumstances) introduced duplicate secondary attributes when using multiple secondary foreign keys.
This bug has always been there. It just came up for the first time now.