PaperAbstract cleaning.

Adde some documentation.  Also, there were some fields that were public
but also had accessor methods.  To avoid redundency (and reduce the
number of ways modifying PaperAbstract can break things) I made the
fields private.
More documentation for PaperAbstract.
Removed a confusing PaperAbstract constructor.

It was only used in one place, and had an almost identical signature to
a constructor that did something completely different.
More documentation of PaperAbstract.
Wrote some PaperAbstract tests.
PaperAbstract: test generateTf and generateTagTf.
PaperAbstract: tests, clarification.

- Rename generateTagTf method to generateTestingTagTf, since it should
  only be called for test documents.
- Document the way generateTf and generateTestingTagTf update the Term
  array passed to them.
Test PaperAbstract.getTrainingTfAsSimpleMatrixRow.
PaperAbstract (failing) tests.

Tested getNorm and similarity.  The new tests fail, because
PaperAbstract.getNorm() returns sqrt(2) times the norm right now.  I
want to discuss the impact before changing it to be the correct norm.
Test getCombinedTf and update doc.

(When I made the test, I discovered that getCombinedTf doesn't do what I
thought it did.)
Document and test PaperAbstract.freqMap.
Test PaperAbstract.equals().
More documentation for PaperAbstract.