Fix project bv curl conf

Hi,

Here's a new function to handle the Dirichlet boundary conditions for FE_Nedelec elements. It uses a fairly simple L2-projection method to compute the degrees of freedom on edges and faces. Rather than replace the current function, I've given it a new name as this may be less efficient on geometries with rectangular faces.

This also doesn't solve the issue when the face orientations are not of standard type.
Here and below, I think you can make a significant number of variables `const` since you initialize them right at their declaration. This makes the code easier to read (the reader knows right away that the variable will not be modified again later on) and helps the compiler find programming bugs.