Add diff-list-set.py

Compares two files which both contain lists of sets.
Lists are the different forms, and sets are for parallel forms.

Usage example:

$ ./diff-list-set.py nouns.6forms.csv nouns.6forms.gf.csv | cut -f2 | sort | uniq -c | sort -nr
  13853
   1507 2-3-4-5-6
    524 3-5-6
    425 3-4-5-6
    267 2-3-4-5
    251 5-6
    219 2
    186 6
    112 2-4-6
     58 3
     55 2-4-5-6
     50 2-3-5-6
     32 2-4
      9 3-5
      1 4-6
      1 2-3-4-6
      1 2-3-4

E.g. "5-6" means that the 5th and the 6th list element differ.

(The accuracy of the 1-arg noun paradigm is currently 13853/17551 = 79%.)