add time_series tutorial

[adinhobl]

Adding a tutorial for time series that is based on the Tensorflow time series tutorial. So far, I've done it in vanilla Flux and MLDataPattern.

In general any help on how idiomatic Flux/Julia it is would be awesome. I was planning to use Literate.jl or Weave.jl and talk about the Flux specific aspects of training models and then either make it into a webpage like SciMLBenchmarks. Reference to earlier discussions.

A question I had (among other questions listed at the top of the time_series.jl file) was whether it should use other FluxML training tools. We could do the examples in vanilla Flux, and then use those tools to show how easy they make it, but maybe I'm getting ahead of myself.

I'm done with things up to the 1-D conv, which I had some trouble on. There may be some pointers here.

Please feel free to pick this up and improve/work on it! I won't have as much time going forward to finish it in the next few months.