rate.InterpBinnedArray:  fix 2d interpolation w/ scipy

- had the array transposed in the interp2d initialization

- note that the behaviour of interp2d and the inline fall-back interpolator
  are different in the middle of a bin so this patch also removes that
  example from the docstring to get the unit test to pass
InterpBinnedArray:  add 1-d example to documentation

(enables unit testing of 1d interpolation)
rate.InterpBinnedArrray:  expand 2-d example

- don't use the same co-ordinates in both directions.  tests that we've got that the right way around in the interpolator as well