Create a function to calculate nodal forces from a traction along an arbitrary edge

Right now we are manually calculating and prescribing nodal forces in the config files. This is easy enough to implement for first-order elements, but becomes more of a pain for higher order elements.

A function which accepts the mesh, traction, and the equation of a line, and returns the vector of nodal forces and corresponding nodes/DoFs would be a nice quality of life enhancement, and make debugging easier.