vectors.py: More documentations and more fine control over
vector and skyrmion chirality and orientation.
Also fixed create_vector_singularity.
colors: Added a namespace for colormaps
(instead of a dictionary) to ease access to the maps.
CMAP_CIRCULAR_DEFAULT is therefore deprecated now!
plot2d: significantly simplified the symmetric color map handling with
DivergingNorm (which was built for exactly that).