wrong arg contours along branch cuts

[nschloe]

E.g., the green line at negative real for np.log:

[nschloe]

Perhaps we'll have to roll our own contouring. It's not too hard at first sight: Just split the domain into little quads, find out if a contour passes through it, and if yes add it to the LineCollection. It gets harder if we want to draw the paths as consecutive lines, see https://cstheory.stackexchange.com/questions/50609/exact-path-cover-for-undirected-graph.

[nschloe]

Done via https://github.com/nschloe/mplx.