Merge pull request #65 from paulvojta/master

The correct answer to this problem is a function, not a function up to a constant
Add missing solution variable.
Correct formatting.
Eliminate correct but imprecise options.
Improve answer checking, problem language.
Clean-up code.
Update correct answer format, clarify instructions.
Correct solution text and answer for one version of problem.
Correct typo in problem statement.
Fixing tags
Changes from OPL 3 workshop, Charlottesville VA
Updating taxonomy files
But with weighted answer grader, taxonomy2 update
Tiny change.
Fixing cross-listing in Taxonomy2, fixing a duplicate
Merge branch 'master' of github.com:openwebwork/webwork-open-problem-library
Fix typesetting error
Correct problem wording and solution text.
Change parameter values to ensure given function is a valid density
function, improve formatting.
Fix opl tag