wt.artists.Axes.contourf() is broken due to TypeError
`wt.artists.Axes.contourf()` attempts to add a `tuple` and a `list` together when calling `matplotlib.axes.Axes.contour()`. See [this line of code](https://github.com/wright-group/WrightTools/blob/development/WrightTools/artists.py#L126).
diff --git a/WrightTools/artists.py b/WrightTools/artists.py
index fc74788..bfd0040 100644
--- a/WrightTools/artists.py
+++ b/WrightTools/artists.py
@@ -121,7 +121,8 @@ class Axes(matplotlib.axes.Axes):
         contours = matplotlib.axes.Axes.contourf(self, *args, **kwargs)  # why can't I use super?
         # fill lines
         zorder = contours.collections[0].zorder - 0.1
-        matplotlib.axes.Axes.contour(self, *(args[:3] + [len(contours.levels)]),
+        levels = (contours.levels[1:] + contours.levels[:-1]) / 2
+        matplotlib.axes.Axes.contour(self, *args[:3], levels=levels,
                                      cmap=contours.cmap,
                                      zorder=zorder)
         # PathCollection modifications
['WrightTools/artists.py']