Update Value/AnswerChecker diagnostics output.

Use plots.pl to create the graphs instead of the old graphmacros.

Remove use of tables for formatting. Use bordered divs instead.
Cleanup some of the HTML use with this as well. Space the graphs
vertically, to work better on smaller screens or in the problem
editor.

When finding test points, remove adaptive parameters from
the list of variable names, as these can make it so the
test points shown on the graphs are incorrect.

Only use of a table is for the table of test points, which now
uses row headers, padding, and no longer uses blank cells for
spacing. Multivariable functions have the test points for each
variable in their own row instead of showing them as a point.
Only variables used in correct or student answers are shown.

Author: somiaj