Fall back to linear interpolation when interpolating in short distances (#388)

[eggpi]

Hi, I got affected by issue #388 and thought I'd give the linear interpolation solution a try, since it is suggested in the discussion there (and the Wikipedia article linked in the code), and also seemed to give sensible results after trying it out for a couple of examples. Does this seem like a reasonable approach?

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[stephenmcd]

Great stuff, thank you.