contours descriptor should allow range objects

[sgdecker]

Another issue (referenced below) also discusses the contours setting, but in a slightly different way. What I'm looking for here is the ability to not have to coerce range objects to lists. While forcing the user to use list() isn't that terrible, it is a pain point that doesn't seem like it should be necessary.

So,
fills.contours = range(0, 40, 4)
should work, where fills is a (Filled)ContourPlot object.

• I'm not a huge fan of the current "contours" set-up. For GEMPAK users they have been used to giving a contour interval if
  only a single number is given, not the number of contours to plot. Would be great to have the ability to set just a contour
  interval or a tuple of (cint, start, stop) or something similar

Originally posted by @kgoebber in #1016 (comment)

[dopplershift]

Should be pretty easy to add Instance(range) to the trait in ContourTraits as well as add a validator to call list on the range there so that the subclasses have the right value available.