Remove duplicate getindex and setindex! fcns

[lrennels]

There are several places in timesteparrays.jl where two functions are parameterized by FixedTimestep or VariableTimestep, but have the exact same body, and thus could be generalized to be parameterized by TS where {TS <: AbstractTimestep}. We should go through and clean this up, but with an eye towards potentially ambiguous or slightly more specific functions that do parameterize on a specific Timestep temp and thus could cause problems with dispatch.

@ckingdon95 I'm moving some of our work from #580 here