Add affine mapping for edges and EdgeBasis

[kinnala]

Discussion at: #1171

This is a proof-of-concept which runs successfully only the following test:

from skfem import *
from skfem.assembly.basis.edge_basis import EdgeBasis

m = MeshTet().refined(4)

ix = m.edges_satisfying(lambda x: (x[0] == 0) * (x[1] == 0))

ebasis = EdgeBasis(m, ElementTetP1(), edges=ix)

basis = Basis(m, ElementTetP1())


f = basis.project(lambda x: x[2] ** 3)


@Functional
def integ(w):
    return w['f']  # f(z) = z^3, integral of z^3 should be 0.25


print(integ.assemble(ebasis, f=f))  # 0.24992399019878747