Drop `AbstractArray` inheritance?

Now that #206 is implemented and the main way to use Interpolations.jl is with function call semantics, I think we should evaluate what value `AbstractArray` semantics provide, and whether maybe we should drop it completely.

My only strong opinion here, is that we should try to minimize the complexity of the package (it's already quite complex...), so if there are features or functionality we can drop without disappointing our users, I think we should.

There are already a couple of places where dropping or changing the semantics of `itp[x]` have been mentioned, e.g. https://github.com/JuliaMath/Interpolations.jl/pull/238#issuecomment-425417528 and #227.