Package to support SVG graphics and file graphics in WWPlot system (PG portion)

Mike,

I think the changes to PG and WeBWorK2 to support SVG graphics as well as file graphics in the WWPlot system are ready for others to try out.

This is the pull request to merge the changes from my copy of the develop branch of pg into the main openwebwork/pg develop branch.

Sincerely,

Bill