Fix bug in colon indexing to timestep arrays

[lrennels]

[corakingdon]

@rjplevin @lrennels can someone give a one sentence explanation of this one?

[lrennels]

@ckingdon95 We thought that there was a situation where indexing into a TimestepArray with a : in the ti index caused a deprecation warning (and thus eventual error), but we want to be able to use the :. As it turns out I think it was just a bug in the testing suite not in the code itself. Make sense?