up #252
implement the 2D and 3D case of the magnetostatic example
up #252 (and #251)
 - implement edge tangents on the tetrahedron
 - implement Nedelec polynomial set of the first kind in 3D
 - fix dual construction in 3D

the initial convergence test in 3D indicates that everything seems all right
with the lowest order element @feelpp/po
see doc/manual/magnetostatic
up #252
 - add nedelec in 3D (lowest order)
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop