Added project_boundary_values_curl_conforming_l2() to VectorTools for
use with FE_Nedelec elements.

Also added a code to tests to check it works correctly.

This new function uses an L2-projection on edges and faces (only edges
in 2D, both for 3D) to compute the constaints. The original function
project_boundary_values_curl_conforming() has problems with
non-rectangular faces, this one does not. The function assumes that the
edge/face orientations are standard.

The test shows that project_boundary_curl_conforming_l2() works on a
distorted cube (non-rectangular faces) for a simple test case
(polynomial function).