Change variable name `step` to something else

[charleskawczynski]

In several places, (for example here), step is used as a variable name, but step is a Julia intrinsic function:

help?> step
search: step StepRange StepRangeLen CompositeException RoundNearestTiesUp InvalidStateException isconcretetype

  step(r)

  Get the step size of an AbstractRange object.

  Examples
  ≡≡≡≡≡≡≡≡≡≡

  julia> step(1:10)
  1

  julia> step(1:2:10)
  2

  julia> step(2.5:0.3:10.9)
  0.3

  julia> step(range(2.5, stop=10.9, length=85))
  0.1

So, we should probably not overwriting it to avoid confusion.

[glwagner]

iteration is a possible alternative for such a variable.