Add lots of comments. Some minor substantive changes too:

Add lots of comments.  Some minor substantive changes too:

* casereader-filter.c (casereader_filter_destroy): Make sure to
write all the remaining excluded cases to the casewriter, if any.

* caseinit.c (init_list_destroy): Rewrite.
(init_list_clear): Ditto.

* casegrouper.c (casegrouper_get_next_group): Always set *reader
to null when returning false.