Add Bloch wall and Néel wall profile.

The profiles for a Bloch wall (bloch_wall(xyz, thickness=10, x0=0, domain_polarity=1, wall_polarity=1)) and a Néel wall (neel_wall(xyz, thickness=10, x0=0, domain_polarity=1, wall_polarity=1)) have been added to the vectorfields submodule.

The profiles are implemented according to

    \mathbf{m}_\mathrm{Bloch} = \begin{pmatrix} 0 \\ p_\mathrm{D}  \cos(\theta) \\ p_\mathrm{DW}  \sin(\theta)  \end{pmatrix} \qquad \mathbf{m}_\mathrm{Néel} = \begin{pmatrix} p_\mathrm{DW}  \sin(\theta) \\ 0 \\ p_\mathrm{D}  \cos(\theta)  \end{pmatrix}

with

    \theta(x) = 2\arctan\Big(e^{(x-x_0)/\delta}\Big),

the domain- and domain-wall polarities p_\mathrm{D} and p_\mathrm{DW}, the wall position x_0 and the wall thickness \delta. The implemented Néel-wall profile is only an approximation, namely a Bloch-wall profile, with rotation perpendicular to the wall plane.