Merge pull request #89 from paulvojta/master

Fix misspellings and add units
Set domain to [0.25,5] since x<=0 is not allowed
Fix bug 3177, two typos.
Fix bug 3178, and added units
Fix bug 3179
Fix bug 3180, wrong answer.
Fix bug 3183
additions and edits to PCC files in Contrib.
one small bug in a Rochester OPL problem
Merge branch 'master' of git://github.com/openwebwork/webwork-open-problem-library into pccProduction
Merge pull request #90 from Alex-Jordan/pccProduction

Pcc production
Fix bug 3188
Merge branch 'master' of github.com:openwebwork/webwork-open-problem-library
Fix bug 3189
Fix bug 3191
Fix bug 3192
Add "weight" to part (d) of solution.
Merge pull request #91 from Dick-Lane/master

Add "weight" to part (d) of solution.
Remove 5 from correct answers

The additive identity is (0,1), so there is a zero vector and item 5 should be removed from the list of incorrect answers.
Remove 6 from correct answers

(x,y)+(-x,0) = (0,1), so additive inverses exist.