Add plot type for nematic tensor fields

This would essentially be a vector plot based on `matplotlib.quiver` of the nematic direction and its strength, which could probably reuse a bit of code from the `DataFieldBase._plot_vector` method. This function would also only work for 2D Cartesian grids (for now) and it probably needs to assume that the 2D tensor actually describes a nematic (what would happen if we use a general field?).