Add interpolate_curve operation and Curve interpolation plot option

[philippjfr]

Recently I prototyped a Steps Element which would plot changes in the y-values of a Curve as discrete steps. It was pointed out this really just represents different interpolation behavior for Curve plots. I think this is a good suggestion, and I'd like to start using it for a project I'm working on. I've added step_curve as a separate operation so you can also plot steps when datashading simply by applying it to the Curve before datashading is applied. Here's what it looks like:

%%opts Overlay [legend_position='right' width=600 toolbar='above']
arr = np.random.rand(10)
hv.Overlay([hv.Curve(arr, label=intp)(plot=dict(interpolation=intp))
            for intp in ['linear', 'steps-pre', 'steps-mid', 'steps-post']])

[philippjfr]

@jordansamuels I've now implemented support for stepping on Curves.

[philippjfr]

Bit perplexed by the persistent python2 test failure. Importing from import hv.operation in plotting somehow changed some axis ranges in two bokeh chart plots (Bars and BoxWhisker)?

[jlstevens]

I think an interpolation plot option would be very reasonable but I think the implementation should be generalized further.

I would like to see CurvePlot pass the interpolation parameter to a general interpolate_curve function (or similar) that happens to support the various step options. Right now this operation would support 'linear', 'steps-mid', 'steps-pre' and 'steps-post' but you could imagine it being extended to support additional types of interpolation in future.

[jlstevens]

I would consider generalizing this to an interpolate_curve that supports the same interpolation options as CurvePlot - currently this would just mean adding a 'linear' option that does nothing. I would also consider making the functions above into classmethods.

[philippjfr]

Updated, ready to merge when tests pass.

[philippjfr]

After rebasing with master this now seems to work okay, ready to merge.

[jlstevens]

I guess we could have bezier/splice interpolation in future by sampling a spline into a Curve - although this would make the sampling rate into a free parameter (though I suspect plotting libraries often do this behind the scenes). I wonder if we could have a Spline element as a subclass of Curve but I would need to think about it more...

Merging.

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