Cell field evaluation at arbitrary points

[principejavier]

@santiagobadia , @JordiManyer please review this PR, I intent to use it to extract line cuts in 3D models in a simple way, e.g. by

x0=Point(0.,0.5)
x1=Point(0.,0.5)
n=10 # number of points in the cut
t=[(i-1)/n for i=1:n+1]
x=map(x->x*(x1-x0)+x0,t)
u=uh(x)

will provide a cut at y=0.5.