Check settings before writing label pairs.
Refactor method name.
Change output file names for clarity.
Add setting and output file for writing singlets.
Add output file for writing singlet counts.
Change output directory name.
Code refactor.

Major refactor of naming conventions used in the code that generates
label co-frequency and singleton counts.  Purpose of the refactor is to
maximize similarity to the language used in Nigam’s research paper,
e.g., cooccurrence -> cofreqs.
Unit test file rename.
Refactor class and method names.
Add unit tests.