Add PL Graph Constructor

[chris-rau]

We propose pl-graph-constructor, a custom interactive element for PrairieLearn that enables students to construct graphs in a drag-and-drop interface and converts the student-entered graph into a DOT-language form to facilitate autograding.

The element provides the student with an empty canvas in which they will construct a graph. This graph can either be graded using pre- or self-defined methods, or the canvas can serve as a sketching element for the student.

In either case, a student can create nodes simply by clicking on the location at which they want to create the node. Edges can be created by selecting the node at which the edge originates and dragging toward the next node.

[armandofox]

hi - can you tell me why there are changed files inside elements/pl-faded-parsons, since that element has nothing to do with your project?
if it is a matter of accidental whitespace being added etc, can you cleanup your PR so that these changes are not part of the commit?
thanks

[armandofox]

you shoudl not be changing infoCourse.json since this file is global to the entire STAR repo. please remove these changes from the PR. all of your project's changes should be limited to:

• the courseInstances/ subdirectory corresponding to your project
• the questions/ subdirectory corresponding to your project
• the elements/ subdirectory corresponding to your project
  No other changes to files above that level should be necessary, unless there are specific files that must be added to serverFilesCourse for your element to work.
  in particular, in your PR i also notice changes to the toplevel README.md and to the elements/pl-faded-parsons. (the latter may be an artifact of the repo being out of date with respect to yours.) unless you specifically had to modify these files in order to make yours work, which i doubt, please remove those files from the commit as well.