Fixed bugs introducd by merging PR #2732.

Printing of vectors and dyadics were broken when PR #2732 was merged due to
some missing imports. There were no tests for this, so I've added a couple for
pretty printing. I have some tests for latex printing in PR #2772 that has not
been merged yet.
Added import to dyadic.py.
Added tests for dyadic prettying printing.
Merge branch 'master' into broken-vector-printing
Added back code from PR #2728 which was removed in PR #2732.
Added tests for latex printing, expected to fail.
Prettier line.
Merge pull request #2862 from moorepants/broken-vector-printing

Fixed bugs introduced by merging PR #2732.