Prelim. support for user-defind vector field functions
Merge branch 'master' of https://github.com/UMLComputerGraphics/GraphicsProject
Adding compiler switch for exprtk on/off, for people with slow computers. use make EXPRTK=true to build with user-defined functions support. There is currently no interface. I will leave that up to Nick St.Pierre and Franck Kamayou.