ChainRules usage isn't tested

[oxinabox]

#84 should have failed as DoesNotExist is no longer defined by ChainRulesCore (renamed to NoTangent) (fixed in https://github.com/PumasAI/DataInterpolations.jl/pull/85) but there are no actual tests of that directly. Which makes https://github.com/JuliaDiff/ChainRulesCore.jl/pull/292 kinda pointless.

It should be relatively straight forward to set them up using ChainRulesTestUtils.

[ChrisRackauckas]

Oh yes, they are all tested directly: https://github.com/PumasAI/DataInterpolations.jl/blob/master/test/derivative_tests.jl, but not through the rrule. That's a silly omission.