Initial positions of added residues are more intelligently chosen

I tested this on the 19 files that previously failed every time.  With this change, 14 of those files succeeded at least once, and 7 of them succeeded every time.

I then let the full test suite run overnight.  It got through 154 files.  Out of those files, 94% of all tests passed (compared to 89% before), and 80% of files passed every time (compared to 69% before).  There were 36 files that passed more often than before and 17 that passed less often.