Removed the function make_sortkey_unique().
Added the 'is_nested' keyword to the template_substitution() function to allow nested special templates and not just nested regular templates.
Renamed fillout_implicit_loop() as fillout_implicit_indices().
Changed template_substitution() function's "is_nested" keyword to "templatekey", which is more flexible.
Added the argsort() function, and changed from 'sortdict' to 'sortlist'.
Modified test4 in the testing suite to use a negative year in one entry to test code behavior.
Added a sortkey template to test4 in bibulous_test.py that uses .uniquify().
Fixed test2.bbl that had errors due to an 'is_singular()' implementation bug.
Removed the '.remove_leading_zeros()' operator, in preparation for replacing it with '.zfill()'.